I think the beginning of Chasm Logistratum is a favourite location (Looks like this) for these kind of penances. Particularly so as you can quickly do it, then restart the map.
Problem is that you might have to find someone to go in to a private game with you as otherwise your team mates will all rush through this first section. Guess you could let them go, but you’re trolling them a bit if you do.
Anyway ~ if you drop down in to that first long corridor and wait, then eventually a horde will arrive. At least it used to. Block, block, block, to let them all group up. Then throw your Ogryn bomb off the wall so it lands in the middle. Helps if you spec up the extra radius talent.
Bit dull, but it should work. It’s also a good location for doing the Bull Rush penance (knock down x enemies with your charge).
As it happens, I’m also trying to do the bomb penance. But since I’ve a few that I’m doing at the same time, I’m just trying to do it more organically. Think I’m on 15/25. It is an awful penance as it really isn’t teaching appropriate use of what is a great elite-pack clearing tool.
*edit: I should add, this particular area is quite sympathetic to lower level players upscaling to a tier4 or tier5 map, as you pretty much just get horde, not specials or elites. Of course, RNG sometimes so … sometimes it’s not going to be pretty. But as a general principle, I think I’d just play normally until level 30 and tier 4 maps, then go hunt penances if that’s your thing. You’ll tick off progress to many just by doing that, and it’s probably a more efficient use of your time.