I thought Mendez was one of the easiest. Although this cant be done on pro, on normal when he has his Extendo -Spine -Omatic i just load him full on any grenades i have. Once he's in his swinging -like -a -child-on -monkeybars stage i just pump him full of TMP bullets, if i run out just shove a shotgun in his face a few times when he falls. That should do it.
The Hardest for me wasn't actually on the full campaign. Its super salvador. When i first unlocked mercenaries and picked leon on waterworld i heard a chain saw, i thought great just what i need, a dr. salvador to ruin my day. Then i see this gigantic potato sack man with a dual chainsaw swinging it around like a spaz. The only way to get any damage on this guy is to get right in his face and shove the riot gun in his face. NOT COOL FOR A BEGGINNER like i was when i first did that.