Against the Cremator, as the Acrid, you could always use your 2nd and 4th abilities against it when it charges you, that should always hit it. I think your 4th ability might actually be able to do damage to it even if you hit it from behind, because of how it works. Other than that, maybe put some poison in the way? Get a few hits on it when it charges, drones are always helpful, and maybe a good active item or something. Other than that, yeah you're gonna face the problem most melees do against that thing.
EDIT: Also, I finally found the secret stage today, that's actually quite a hilarious little area.
The miner unlock, are there only two layouts to the magma caverns? In that game, my first pass through the caverns I had the layout with the pod in the lower right, past the lava. I could even reach it because I had the... jump jets, I forget what they're called (not the rusty jets that make you jump higher, but the ones that allow you limited flight with the gas meter). But I couldn't activate it like I expected. What did I miss about it?
Edit: Well, never mind. Despite this being the third or fourth time I've been to the pod, this was the first time I saw Direseeker.
Well, I guess you unlocked him, that's good. He's a pretty strong guy who has a lot of mobility (the item you were talking about was the Photon Jetpack, which has the trolliest freaking unlock ever).