I'd be trying to exploit vulns, yes. 4 paths, go lightning/fire/ice/(dealer's choice) The choice being that I can see reasons to take air or ooze or even earth. I'd probably avoid water due to no low level damage spells.

With those first three paths you have vulns on three fairly popular races, Fire/Arials/Felar. Elves/D-elves at that level are exceptionally low on HP and probably not packing much in the way of svs, plus due to their EXP penalty you're going to have a 5-8 lvl advantage over them typically. Your toughest foes are probably going to be other giants/svirfs/dwarves/duergar, and even then I'd just break out ice/fire spells and maybe a flight/diminution potion/enlarge (depending on the situation) and try to get the jump on them.

Be a human. Lvl 20 human with 10 ice affinity invis rolls up on lvl 12 fire giant lvling on elves/orphans/voralian commoners? That fire giant is dead in one shot. Won't even know you're coming.

I'd personally go goodie(even thought it cuts down on your range), try to get an EARLY induct into fort. All of a sudden you have prot_evil, or 25% more HP on a stick, and access to fort gear train. God forbid someone hits you up with a ragged cloak. (Ragged cloak + invis pre 25 is just overpowered). Commence open season.

There'd be some cherry picking, but as a human your range is so favorable, plus as an invoker, plus almost no one at that level has any sort of svs so they're taking maximum damage...I mean, it's nasty. Really nasty. Sure, you're going to die occasionally to the hero decked ass beating warrior who lands a bash or who you underestimate, but for the most part you're killing a LOT compared to how much you're dying.