I've played the entire game several times, and have played nearly every optimized build there is to play, and these are my thoughts on spells that is so far beyond normal in terms of how good they are.
In this.... (complaint? guide?) thread, I'll list each of the spells that I think are overpowered, and why. If you are having trouble with the game getting stuck at tough fights, this will (hopefully) make you power through without robbing you of the feel of accomplishment that following detailed guides does me.
Sorted by vendor sell requirements, and how broken they are for the level you can acquire them (a few may be wrong tho, it's been a while since I played a physical team).
Skills tagged with [SG] are shotgun spells that shoot aoe projectiles at each enemy in range. These spells deal devastating damage to clumped up groups of enemies as each projectile will hit all the enemies in the groups, which basically means you multiply the damage of the spell by the SQUARE of numbers of enemies as the total damage dealt. I think Larian balanced most of these spells without considering the shotgun effect, or at least severely underestimating it.
Lvl 1-3
These are the spells you have access to from the very start of the game. Early game gameplay features a lack of armor on the player's side, and thus being able to quickly kill the enemies or avoid damage altogether plays a much more important role than the beginning of most games.
Tier "OP As Heck"
Bow/Crossbow Auto Attack: disgusting damage, huge range, 0 cooldown. What more could you want?
Chameleon Cloak: I'm not talking about the "cloak the low hp companion" chameleon cloak, I'm talking about the "cloak every single guy on the team" chameleon cloak. You basically get full AP if you wait a couple rounds, then another full turn/half turn if you delay your turn on the round you decide to break stealth. Especially powerful on lonewolves, as it's basically 2.5 turns worth of free AP.
Corpse Explosion: 1AP for over double the standard spell damage in a moderate area. Only downside is the reliance on the presence of a corpse, which is a very easy problem to get around. As an added bonus, it's undodgeable physical damage.
Throw Explosive Mine: 1AP for nearly double the standard spell damage worth of fire damage in a moderate area, with a delay so you can set it up before battle, then have it off cooldown by the time you trigger it with some other spells. As a drawback, you need to detonate it with another spell before the end of turn, as they do reduced damage if triggered by anything else.
Tier "Very Stronk"
Adrenaline: It's utility that literally every build would benefit greatly from, enough to spend the skill point in scoundrel just for it.
Elemental Arrowhead: Huge damage buff of something close to 30% of weapon damage, not much else to say.
Backlash: 1AP for mobility AND nearly a full auto attack worth of damage with guaranteed backstab, highly AP efficient.
Lvl 4-8
By now enemies start spawning with much higher hp in comparison to the players. Just bulling through with raw damage is no longer feasible in most cases. Most of the spells in this level range aren't much better than lvl 1-3 spells, just more spells to round out builds so that you have less downtime forced to use autoattacks
Tier "OP As Heck"
Supercharger+Summon Bloated Corpse: This is the summoner must have. The damage from detonating a supercharged bloated corpse is ridiculous. If you focused all but one or two of your skill points in summoner, just the damage of the suicide explosion would be around 2~3 times what a normal 2 AP spell does for damage builds, all for 2 AP total. As an added bonus, it's undodgeable physical damage.
Tier "Very Stronk"
Teleport: A truly wonderful spell. While it does decent damage on its own, often to 2 enemies, what makes this spell so powerful is that you can stack multiple enemies inside the area of your attack spells. This is how you continue bulling through with raw damage. With teleportation gloves and one of your companions getting aerothurge 2, you are nearly guaranteed that in most fights, you start the second round with less than half the enemies left alive and/or un-CC'd after your initial burst of AOE spells. Teleport also hits as hard as normal 2 AP spells if you go intelligence+warfare.
Tactical Retreat: Yet another wonderful spell. It costs 1AP for a very long jump, and the haste means you get that 1 AP back the next turn in addition to the increased movement speed during those two turns. It is especially powerful for ranged builds that can abuse this spell to not only get highground bonus damage, but also waste more enemy AP than you spend when it chases you against melee enemies. Absolutely worth the 2 points in huntsman for any ranged companions, ranger or not.
Lvl 9-12
You are adrift on Driftwood. So many opportunities around many levels above you. Unless you really know the map, this is when you'll find a lack of same leveled encounters to fight. Faced against enemies with extremely high armor/vitality because you are underleveled, you NEED an edge.
Tier "Game Breaking"
Deploy Mass Traps: Remember the throw explosive trap that was so OP earlier? Well, here's an even stronger version. With 3 AP and 1 SP, you throw 4 traps that are each as powerful as a 3 AP spell. Only drawback is that since it costs 3 AP (2AP if elemental affinity'd), you need to plan your turn well to detonate it with another spell before the end of turn, as they do reduced damage if triggered by anything else. #3 broken spell in the game.
Mass Corpse explosion: Remember the corpse explosion that was so OP earlier? Well, here's an even stronger version (deja vu). With a wopping 1 AP and 1 SP, you can detonate EVERY corpse in a 13 m radius. Even if you don't cheese by stacking corpses before a fight, chaining this spell after a normal corpse explosiong that usually creates another corpse, (then again if you have skin graft) usually ends the fight there and then, despite enemies spread out somewhat. The damage is just that insane. Note: this spell is about balanced with most other spells not listed in this OP list if you detonate only 1 corpse, in terms of its overwhelmingly powerful raw physical damage. #2 broken spell in the game.
Tier "OP As Heck"
[SG]Dust Blast: It shoots a much more damaging Throw Dust at each enemy in range for 3AP 1SP. Deals the normal 3AP spell's worth of damage EACH, with good enemy clumping. A straightforward shotgun spell that does earth damage and blinds enemies. Only reason it's not in Game Breaking is because of the high AP cost on a skill tree that couldn't decide if it wants to deal poison or earth damage.
[SG]Bloodstorm: It's the only 3 source spell that you can get this early in the game by combing spellbooks. It basically launches a small aoe 2AP spell's worth of damage at up to 5 enemies in its range every turn for 3 rounds. While one of the most devastating spells in the game, it has the triple drawback of high cost, need enemies to be clumped to really shine, and the damage is spread through 3 rounds (2 if you use delay turn, it'll trigger both times you start your turn).
Nether Swap: 1AP for switch positions. With lucky positions before swap, it can fill the role of a cheaper Teleport, but even in the worst case scenario, coupled with tactical retreat, can be used to position enemies anywhere in range.
Tier "Very Stronk"
Ballistic Shot: massive MASSIVE single target damage if you get a highground shot at 15+m for just 2 AP.
Deathwish: the golden spell of cheese. If you manage your initiation well and/or have a good burst damage spell set up, you can delete enemy(s) on the very first turn. Only reason it's not up there in the OP as Heck tier is that it requires so much setup pre-fight for this spell to shine that I don't think people who have trouble with the game will want to use this spell.
Pressure Spike: 1AP spell that does pretty good damage. Only in this list due to the difficulty of setting up Aerothurge elemental affinity, otherwise it's not any better than a 2 AP spell.
lvl 13-15
Small window before you get everything. This phase is mostly where you round out your build with utility spells and going for more stats.
Tier "Very Stronk"
Closed Circuit: Huge damage with just 2AP 2SP that stuns just about anything in range. Only problem is you have to get close to enemies, and that's not really healthy for mages.
Ice Breaker: For a enemy surrounded by ice surface, this 1 AP spell deals something like double the damage of standard 2AP spells. Situational as in you need large amounts of ice surface for it to work, but the damage really pays off.
Grasp of the Starved: Massive damage to targets standing in blood at the low cost 2AP 2SP. Only problem is, large blood patches is hard to get (Raise Bloated Corpse moves after you, Blood Rain costs 2AP and Blood Storm uses up all your SP). As an added bonus, it's undodgeable physical damage.
lvl 16+
If you spam those 3SP spells, this phase of the game gets a lot easier...for ranged builds, that is.
Tier "WTF Larian"
[SG]Pyroclastic Eruption: Shoots a projectile with huge AOE that deals something like 3~4 times the damage of normal 2AP spells, each. AND THEN IT SHOTGUNS. Holy mother of Lucian this spell is broken. You can literally oneshot anything that's not earth immune in this game with this spell, including the lvl 20 boss with 80% earth resist (Kanon), which I did at lvl 18 using a wizard that mostly took pyrokinetic and huntsman instead of geomancer.
Tier "OP As Heck"
[SG]Thunderstorm: basically the exact copy of Bloodstorm, but deals air damage and stuns. Slightly worse in the damage department because physical damage doesn't have to contend with resistance, but the stun means not killing everything on turn 1 doesn't hurt as much.
Meteor Storm: Very high damage AOE burst that costs 4AP 3SP. Hits the hardest to targets in the dead center, about 4~5 times a normal 2 AP spell, and falls off towards the edge of the effects.
Tier "Very Stronk"
Hail Storm: Very high damage AOE burst that costs 4AP 3SP. Hits the hardest to targets in the dead center, about 3~4 times a normal 2 AP spell, and falls off towards the edge of the effects.
Arrow Storm: High damage AOE burst that costs 4AP 3SP. Hits the hardest to targets in a decent area in the center, about 2~3 times a normal attack, and falls off towards the edge of the effect. However, since the low damage comes from the significantly smaller impact radius of arrows compared to meteors and hail, the fact that it does physical damage and usually has a higher damage with good bows equipped means that it absolutely destroys giant targets (e.g. kraken), doing something close to 6~7 times the damage of normal attacks.
