If any skill seems mandatory it's the Rogues Invisibility, and only so you can steal tons of gold before hitting level 7 and getting a spell to do the same thing.
Recently I started a game with my gf playing a 2h wielding caster. She's got Man at Arms, Pyro, Hydro and Aero. The premise is access to teleportation, healing and fire spells for the times she cannot charge into combat swinging away with a giant sword (also thick skin and leech for survivability).
I am complimenting this with my Rogue. Dex and high speed (2 speed rings put me at a 9 speed atm) plus the Ranger self teleport means I can deploy behind a mob and maximise backstab to take our casters if needed. I also carry a bow and many arrows in case I need to take out a flaming barrel, or even lay a steamcloud down for the hireling to electrify.
I took out a level 5 guy in town at level 3 without him even getting a turn, thanks to my backstab, high action points and stuns. (my gf and her minion were off exploring something at the time since we split up - usually she does the story stuff while I steal everything in town to fund skillbooks).
