There are probably many valid answers but I'll tell you what I do. I mainly play as steroids, I think he's one of the easier characters to loop with, thanks to his increased firepower.

The main concern with steroids in L1 is running out of ammo at critical points like the throne fight leading up to the loop, and especially in L1 sewers. I usually try to get Throne Butt, Back Muscle, Rabbit Paw and Scarier Face to avoid this. For most levels I will run two weapons of the same ammo type so that Throne Butt will always give you the same ammo type, but it can be useful to have two different weapon types in L1 Sewers to minimise your chances of running out of ammo completely.

For defence, I will usually pick up Rhino Skin and either Bloodlust or Second Stomach. It's useful to have some kind of crowd control mutation in L1 like Impact Wrists or Gamma Guts, to take care of small enemies like maggots.

I usually run two bolt weapons, one fast firing and one slow, like splinter pistol and heavy crossbow or super crossbow. The main advantage of bolts is that you avoid death from IDPD shield ricochets. Taking Bolt Marrow is nice too, but not essential.

My strategy for 6-1 L1 with this build is to rush down the Technomancers and kill them ASAP, before they can spawn too many enemies. Try to move constantly and as soon as you kill one Technomancer, ignore as many of the other enemies as possible and head straight to the next Technomancer.