Good ideas!
Dirigible,
"Get Bully".
Agreed. Bully is fantastic. I didn't include talent choices on the list.
I also didn't include "how to play with the rest of your team", since..it would be quite a bit more writing, and depend on the level of each character, enemies, location, weather, context..etc.
Kurausu,
Good notes! A lot of the advice here is heavily level dependent. Towards the end game, my int was 11 I think--so it really wasn't that bad. Blind takes so little AP that even without it (mage almost always cast it) it's worth having. All of my team had mid-level int. (9-11).
"Charming Touch and Cloak and Dagger are kinda useless for a warrior."
Oh contraire!
Typically, I would have the mage cast Oath of Desecration (but everyone had it), rather than the tank itself. Charming touch was great, and effective, the entire game. My dex is actually pretty high (10-12), and Cloak and Dagger was *fantastic* to use, it's an instant teleport across the screen that used less AP than moving there. It was also super useful outside of fighting. All of this for 1 skill point, and could be used at level 1.
Remember that your success with a spell isn't dependent entirely simply on your Dex/str/int--but those stats relative to the skill level of the spell. So, since they're low level spells--they all got pretty heavy bonuses, not penalties. (Your primary stat - skill level requirement determines the +- bonus by 10% per point)
With the rest of the Man at Arms--I wrote it as 'these are the things you're probably missing' rather than 'this is the comprehensive path'. I presume any tank is using the entire line of man at arms skills, but may not have been using the above mentioned.
Re: Aerothurge.
Some good stuff there. Invisibility is skill level 8, Lightning Strike is 7--and a tank won't have 8-10 Int until later in the game. Teleport is a great idea.

Notably, damage is also dependent on weight..so..try teleporting a heavy object. Become air: also great idea.
Re: Lightning Spell
I wasn't the biggest fan of Lightning, because it was constantly raining out, and causing too much collateral issues.
re: Summoning
I never summoned anything because I thought it was cheap, and fights were easy enough without them.
I think in general, those who think the tank are underpowered, or not effective--are not using spells much to their advantage. A little tweaking..and voila! Death Tank!

-e