-Ghost Pokemon can pass through walls.
-Recruit Level: Very Hard
Also Claydol can float across water (due to levitate). I had Bulbasuar and Piplup, so it was actually easier for me since I had a few water moves to deal with Claydol and Gliscor. If you keep getting KO'd, you just need to be at a higher level.
I'd suggest you be around level 30 when entering Dark Hill. Just go to other places to level up; doesn't matter how many days you take.
You can only evolve after you beat the game. The Luminous Spring is where Pokemon go to evolve, but it's currently messed up by the time/ space disturbances, so it's not working. You need to beat Darkrai and have everything return to normal.
When it's "fixed", you will see Teddiursa go in and evolve into Ursaring (just after your graduation exam). You will follow, but the Spring will tell you that you and your partner cannot evolve.
You need to have beat Darkrai, then recruited Dialga, Palkia and Manaphy. After you've recruited Manaphy, he (it?) will tell you about Marine Resort. Beat Marine Resort, go to the Luminous Spring (I think with Dialga and Palkia, or maybe it's just that you need to have recruited them), and then you will be able to evolve your starters.
You don't have to do all that if you don't want to evolve your starters; you can evolve any other Pokemon after the Spring is fixed, i.e. when you see Teddiursa evolve.
Pokemon Mystery Dungeon Explorers of Darkness walkthrough