4-10, during the "lightning" level, from what others are saying, playing through again now to see. Make sure to have something to blow the fog away so you don't miss it. --- Once again snatching defeat, From the jaws of victory.
There's no fog in level 4-10. Anyways, you can't miss him, since he appears along the others in the zombie preview, and you'll have to trouble beating him since he appears when you're fully armed. It seems beating him there unlocks him in other stages, because he just popped in my Survival Endless game.
How strong are they? I'd suck to unlock them on Survival Endless if they're too powerful! On the other hand... if they're weaker than Gargantuars and their presence makes Gargantuar less likely to come out, it might be a boon instead!