
The slowed movement absolutely guts their damage and they take increased damage. It's been mentioned already, but the Decrepify curse essentially shuts bosses down.

The Pits in Normal will give you loot drops in line with whatever else you found in Act 1 when you just started, not worth farming at all, and you won't get XP either. There are a few side dungeons in the game where nearly any item can drop, but these are all in Hell. The Pits is a fantastic dungeon to farm but that's in Hell mode where it's a much, much higher level. Wherever you are currently at is the best spot to farm since you're fighting the highest level enemies you have access to. It's not very likely that you'll find anything notable in Normal or even Nightmare difficulty.

For where you're at now, just keep progressing forward summoners really don't need much in the way of gear.
