RE4. I'm currently fighting against the Right Hand, and he's a freakin pain. I'm assuming I'll figure out an easy way with the nitrogen tanks, but this is yet to be determined.
Hang out around one of the nitrogen tanks, then tip it over when it gets close. When !alien goes all icy and slow, pull out the rocket launcher you picked up a few scenes back, aim, say something witty, and fire.
That's the easy way, at least. If you don't have the rocket launcher, freeze it, unload however many magnum shots you can into it before it unfreezes, then get out of that room, get to the next tank, repeat and rinse until one of you is dead.