Hey there Filda. Congrats on reaching the endgame.
For bosses i've used a team consisting of 2 prankster whimsicotts with memento, a uxie with t-wave, stealh rocks, and memento, and any pokemon that learn spikes. Then I switch into clefable and setup cosmic power and calm minds, then sweep.
Only bosses i've had problem with this team are Misty and Pumpkin King, in which they lead with a competetive milotic and defiant bisharp. Easy way out of this is to lead with ferrothorn first and leech seed stall them out.
Another good alternative boss pokemon is snorlax. He gets both amnesia (sharply boosts sp def), and curse(boosts atk, def, but lowers speed). Just be careful of crits when using these.
Generally walls with setup moves are good options for bosses.
-Slowbro with calm mind and slack off
-Chansey with eviolite
-Ferrothorn
-Klefki screens support
Hopefully you can make a boss team that you can use consistently. Good luck ^^