This is pretty tough IMO. Both Merged Zamasu and Vegito were said to be only surpassed by initial Jiren who is on part with initial Ultra Instinct Goku who is probably weaker than Kefla given that Omen Ultra Instinct was said to be stronger than initial UI Goku.
I guess my chain goes like this
Jiren 1st try power up >= UI Omen Goku > SSjin2 Kefla > Initial Jiren > Vegito/MZ ~ Spirit Bomb ~ SSjin Kefla
I have MZ and Vegetto still being Beerus tier in the anime, so Kefla gets his butt cheeks rubbed off with ease by either of them. She might be able to tag Giant MZ a few times due to his apparent lack of speed, but she won't be able to finish him off.
Goku's Genki-Dama ought to be stronger than Trunks' Genki Sword that wasted Giant Merged Zamasu, who in turn is likely rivalling SSB Vegetto based on his first mutated form not getting absolutely wrecked by Vegetto like in the manga. Kefla may be able to fight well in LSSJ alone and stomps as a LSS2.