Rook Archer vs Groffling Warplate
Rook with his Archer shoulder upgrades (which are awesome against buildings, btw.) is in deep shit if you can afford to buy the Groffling Warplate.
The arrows come at a fairly high rate and seem to trigger the activate-on-being-hit shield quite nicely.
A near permanent 300 damage buffer is a quite noticable disadvantage for Rook, which I recently noticed to my dismay.