Useful FKG info

For powergaming, your best bet now is a burst team. This calls for a 'weakness up' girl, a barrier girl, plus as many crit/re-action/attack up abilities as you can find. Better if you can sneak in some party re-action abilities, and maybe a '2x skill rate to party if you got hit last turn' girl to really mess with Laeva and Mother, who are currently the toughest bosses anyway. You need heavy ampule/centy investment for best performance, but burst is the go-to team for killing the 'no gimmicks allowed' bosses that have way too much HP and anti-stall measures. Once they're fully kitted out, you can do something to the tune of 1-1.5mil damage/turn consistently, and go higher if your follow-up attacks trigger.

Your other options are debuff and solar, which require very specific builds and aren't really that broken, but they are hilarious to watch. Enjoy doing 2mil damage right off the bat or stalling for 100 turns against raid bosses. Debuff is 3-4 girls with attack down plus a tanky HP recovery type, Kerria is the go-to example. Solar is Dogwood, any four other girls with the '+70% to solar drive power' ability, and five of the '+10% to solar drive' memory. Neither setup needs ampies or centies, beyond the HP drain girl in debuff, so you can focus on your other rainbows first.

An example burst team is Pygmy Waterlily, Wolfberry, Sakura, Valentine's Onci, and a variable slot that usually goes to RG Lily. It does not perform well against bosses weak to slash, but you can cover up if you have more slash types.

A non-maxed burst team still performs well. It just doesn't perform well enough to take out the big names. It's also more flexible than the other options, you can rotate pieces in and out depending on your needs. Laeva for example gives you three freebie turns before putting your party in a washing machine, and barrier won't save you from that. You can replace your barrier with the aforementioned 2x-skill-if-hit ability in this map, while keeping the rest of your party intact.

However, you can make a burst team with almost anything because many rainbows are geared towards burst, while solar is handed to very few girls and is useless unless you commit wholly to it. So pick tickets are your only way to get your hands on that particular setup. That, and Mimosa is really cute and has solar, so you should get her.

For optimal performance, you should probably go for rare abilities like Murasakihanana's extra damage to weakness, Christmas Broom/Silk Tree's party-wide double action, Pygmy Waterlily/Sensitive Plant/Swimsuit Sakura's 'add weakness to non-self weapon types' thing. These are mostly relevant to burst and currently hard to get, though they will probably be more widely available in the future. If you're dead set on debuff, you can also go with Wolfsbane, who has the so-far unique ability to skip enemy turns. Other good picks would be Balsam(very high single target damage), one of :Swimsuit Sakura, Pygmy Water Lily or Sensitive Plant, and either Kerria(you'll need to build a separate team around her) or Silk Tree(her kit is just really good).

For counter parties. Drain+Skill act focused to extend survival, or going full critical and let counter do all the work.
Drain/Skill ones will rely on Hydrangea RG to cap skill act to 100% to continuously attack every turn.

Def+Supercounter is mandatory for such setup, missing a single def buff will drop your damage significant. Regular counter will only be dealing half the dmg of supercounter as well.
Because supercounter is a mechanic that only procs when you "defend", so this makes it very important to increase the defense value of your girls via ampy/centy, equips that reduce damage when defending will help as well.

Aside from Def, crit and weakness are the two other damage multipliers that will amplify your damage.
Best to try and hand pick a few counter girls you want to invest in and work it out.

Sample of a counter party against Laeva: https://streamable.com/ejb3uu
Rainbow Tierlist https://tiermaker.com/list/anime-and-manga/dmm-fkg-6-girls-238325/595243