So... I got the hardcore trophy the other night, and here are my trophy tips:
1) Stay on Alaska 1-1. I had no problem doing veteran the entire time (over double the experience speeds things up). If your gun gets a few levels behind, then yeah, drop to professional or rookie.
You won't get -- sentinels, leapers, flamers, etc. which ups survivability a lot.
2) If you go in a group of 4, you'll get a ton more experience [faster leveling] but you -must- stay together. Get separated and a sudden hoard or whatever will slaughter you... the extra hp enemies have in coop is dangerous.
3) The game -will- start to feel more difficult in the mid and late 20s, especially if your gun is underpowered. I got lucky and a guy who was 30/50+ joined me and basically escorted my character from 26 through 30. [thanks!]
I saw a lot of mid-20 characters join my game and die... thankfully i'm good at dual stick shooters and survived where they didn't.
4) You get a lot of bonus experience for finishing a level, so the best way to farm exp is to finish 1-1 as fast as possible, only stopping for obvious bosses/lewt but not searching the map for them.
5) Bio-specialist. All the way. Healing is essential.
6) Personally, I prefer being host to ensure speed runs for best experience gain, but if you find a good host, that's just as good.
AND....
the best part... once you're level 30, you can now quickly and easily play through the game on rookie and get the 'beat game w/ bio specialist trophy'. I never even came close to dying.
Now I'm just leisurely playing through with a Tank for the plat with a saved 30/something HC and a seriously OP 30/233 saboteur ready for any DLC.