I stopped spawning Bally because the gains were low and it took away the spawn time for Rockster. (about 4 hours of semi active play)
At that time my damage was still low and it took a long time to kill Rockster. What I did, was put all my traps at the front, 3 Laserguns at the T-section and the Gunshots below. Than I rebirthed and capped the Rockster count at 2 so that my turrets could always kill it and they never roamed the map. (Didn't go for shield at that point because of low damage)
Now I have a full row of Lasers at the top of the T-section, all the gunshots below and a trap every 2 tiles. The traps are slow maxed and stop herds of instantly spawned (maxed as well) Rocksters from going past the Gunshots range. To maximise kill time I never buy all upgrades for shield and health. If they need longer to get killed it also reduces money gained. (EDIT: I didn't account for time not spent killing here. Then the spawn time is the limiting number. Best is to balance the number of Rocksters on the field, so that there are always some and never none.)
I tried a round with no health and no shield upgrades. Though I found out, that most of my RP didn't come from monster drops, but from money gained. So I discarded the idea of instantly killing them for more RP.
I also thought about a high health run with damage from mostly traps (% health damage), but I don't have nearly enough traps and I didn't yet check if a monster can reach 0 health with its shield still up. Not taking the shield upgrade is also no option because it gives the most coin upgrade.
For RP progression I never rebirthed if I could not at least afford the coin upgrade from the monsters I farmed at the time. At the beginning I also always saved to get the RP upgrades in the same rebirth and spend the leftovers for upgrades like turrets and RP drops. Now I do have to wait a long time to get any upgrade at all. Focus is still the things that increase RP gain the most (money from Rockster and "RP per level"). I have no clue what "Rebirth award MP" does, but it is maxed anyway. Turrets don't do much anymore because 6 + 1 is an increase of 16% in damage (and you also need to find a place to put it). But I still get them with what is left over. I also invested some RP in the drop chance and amount for Rockster, but it's only 1/30 of the RP I gain.
This is my latest setup