Hmm... I use my thief to eliminate a troublesome enemy after the fight has begun with my tanks. The theif then sneak attacks a spell caster or similar character. I like arterial strike for this, so that I can (theoretically) make them fail a spell. heh heh, they need to have low cons for that though.
The damage though, puts that caster mostly out of comition. The dualwielding assassin then finishes the wizard off and tries to avoid the enemy tanks.