I went down the middle of the tank tree and picked up a few side abilities. By level 12 he's tanky enough for Routine so I'm starting to look at the damage tree.
I currently have Payback Strike, the 8 second invincibility thingie (with upgrade), the aoe taunt + extra guard thingie, and guard itself. Start with the taunt and extra guard, then you can attack a few times and know that everything is going to hit you so guard again and BAM up goes the guard meter even more, then once it gets low hit the invincibility with extra guard generation as you get hit and you swing and stuff and then once that gets low again you're ready for the taunt thing again
so yeah by that point look at the other tree for stuff like faster stamina regen and whatnot, there's some nice utility there for tank legionaire