So I came here. Hope this help(?) If not, ask me what you want to know exactly : )
Whether or not you are interested in picking new companion or just seeking for OP builds.
You are probably good enough in the game already to handle tactician mode without advice.
About the guy that pumped 500h in the game and whatnot, that build sounds like a lonewolf build (or he doesn't take any good mage master spell, which means it's just a tradeoff big burst AoE spell vs high sustain damage with dualwield), but it doesn't matter much as it doesn't define the early game, which is where builds do matter (having an insanely good character isn't required for mid-late game).
Indeed, early game mages are not really damage dealer and they mainly CC and heal/buff (but they can dump fire everywhere which does do some damage though). With 2 points in every school you can pick most good spells from each school, and specialize then (I tend to go with Hydro and Pyro, sometimes Geo; Witch and Aero master spells are meh).
2x mage, ranger and 2h melee is a safe build. If you think you can make a very support-dedicaced mage, you replace the second mage with anything else (in order of efficiency: ranger, other man-at-arms, scoundrel) and get a less safe party but more deadly.
Basically build your mage with 2 points in each school and pick the best spells from each, add him some leadership aswell (and you can dualwield leadership wands to get to 6 leadership very very fast with an helmet and the honest trait on top of it - you will not use the wands offensively). You can also pick one in man-at-arms to get the anti-knockdown/burn ability. Mid/Lategame (starting level 12), get 4/5 in hydro and 4 pyro to get very OP master spells (the big AoE ones) so even your mage support will deal insane damage.
If you feel unsafe, you can give one point to any other(s) character(s) in geo (summon spider, oil) or hydro (heal) or areo (thunder jump, remove stun, teleport) to spread the support in case of emergency. It's cheap and effective. Don't put geo on everyone as you'll get a free summon with Nick.
And if you want to optimize the damage on the other members, check some good guide on crafting (most weapons are easy to craft, easy to enhance and very powerful).
Crafting is quite good for arrows aswell.
Grenades are very good early game aswell, it's worth to have one of your melee get the pinpoint talent.