yeah, the bats there are 51-55, but the lowest that pop in the basement are 53 :/
Bostineaux is alright, but be prepared to be forced to zone at times due to mutiple failed charms, killing chains now and then.
Bombs are generally very safe in a duo, and yes, you do miss on the skilling up while fighting them... but you get to play baseball now and then when they don't blow up
I never actually exped behind the first gate, we just sucked it up in crawler's nest until 53, when we started doing bats > beetles in the basement of garlaige. You'll be able to take on hellmines when you can charm a few more mobs for safety, or you can be ballsy and take them on right away. You will find that parties camping in the passage to the room that they are in tend to get very touchy about bst in the area.
It can also become VERY crowded down there, making it very difficult to find pets. When you can start to take on the jugs, those tend to be very good exp... but pretty slow at 59, trying to hit 60. Parties will try to pull your pets as they are healing now and then also, some will be accomidating of you if you talk with them, other's won't.
goodluck, it's a pain.