I never understood the poke with a single girl thing, except for Seed-Bearers.
Normally I try to hit a RB with a team that will do maybe 10-20% of its total HP because when we have just normal random spawns of maybe typically level 65-75 Seedy Seeds, they die pretty quickly (2-3 people) if I hit them very hard. But when we have a spawnable RB event on, there are going to be more RBs and it becomes imperative to hit them harder.
I can only get to 400-500k on the current Beetles though.
At a best guess, some combination of counters (so multiple attacks responding to boss aoes), enemy ATK down, and direct boost to RB ATK would probably be optimal. The best I could do with units I have would probably be something like Cherry Sage, Bridal Onci, Cyrtanthus, Anemone, and Snapdragon, but 3 of those aren't leveled enough to be useful right now.
I'll almost certainly end up with excess bells; I am not hoarding them by any means but no point in spawning another RB when there's several already up that people are struggling to finish off. Still doing ok towards event goals; I have all jewelry and Sandersonia with all blooms, and 8K in the bank - shooting for about 20K (all manyus and most of the ampys).