Heya, fellow source hunters!

The tact mode is possible, but it is truly hard indeed! I'm happy enough that I did defeat Braccus Rex! :3

I've seen the tips posted by Isu, those are really good. I'll add my own tips, to complete it :

1) Do not, ever, under any circumstance, try to play this mode until you've done at least on play through. Obvious tip, but necessary tip
2) Team composition : Everyone has its preferences. Yet I suggest you don't have too many melees : a lot of ennemies have AoE skills, and if they get more than 2 of your members, this is usually game over.
A rogue /stealth character is often useful. Try to build him to be fast and have a lot of CC abilities. Alternatively, you can use this character to max your charisma & other "utility" abilities. His purpose is not to deal a lot of damages, it is to be useful (see point 6). He is also your "last hope" : when everyone dies, you stealth him, disappear, flee away and come back to rez everyone
An archer is (imo) necessary : the special arrows add so much CC and are so easy to found / craft, you just can't win without it
Janhan is great : he has a lot of spells that are very useful. I suggest you pick him as a mage.
Indeed, you do need a tank. Not some half melee/half magic bullshit. No. A tank. You only level up the defensive abilities, man-at-arms and 1 handed weapon (forget about two-handed weapons. The shield is such a good mechanic).
3) The crafting! Janhan, the mage buddy you can get easily, starts with scientist (+ 1 craft, + 1 blacksmith). Those stats are really important. I know the crafting might seem a little bit boring, but trust me : craft potions, grenades, arrows, enchant your stuff, craft MEALS (the health + stats is indeed important for the fighting). Do not hesitate to check some crafting tables online.
4) The "traits" : Divinity : OS has a good traits system : every action you make towards NPC's has an influence on it. Those traits are not only for the RP : if you look at it, they have bonuses attached to them. Try your best to have the trait bonus fit your character build (some of those bonuses are really good.. Immune to curse, immune to fear..) Think before every interaction.
5) DO NOT get Glass Canon or Lone Wolf. Rip Run.
6) Quite, quite, quite, aaaaaaaaand... Quite! Agro the ennemies from a large distance. get your stealth character in the back of the ennemies. The ennemies will split : some will chase the rogue, other will go to your team. Then, when half the ennemies are far enough, use the TELEPORTATION PYRAMID of the rogue to get to the team and kill half the ennemies. Repeat until win.
7) know the spawns. Some ennemies are stealth, or appear only when you walk a particular path. It is necessary you know the spawn. If you get surprised, you die. The same works with traps (explosive)
8) Do not try to fight every ennemy you see : some are too hard to kill in one shot (I'm thinking about that giant spider in the cave)
9) placement : you have to think before EVERY fight where do you place your characters : hard to reach, easy to lock down an enemy,
10) last but not least : determination. You will die. Again, again and again. Ever Dark Souls didn't prepare you enough for this. If you want to win in this mode, you have to accept that every mistake will make you fail. Sometimes, you won't do any mistake, yet you'll still lose. It's the game, deal with it.

I think that with those informations, you might be able to *perhaps* success in this difficult yet so self-rewarding mode.

Good luck and have fun saving Rivellon again, again and again! <3