I hope to get around to writing a guide on this at some point soon. While the damage calculation is relatively simple, it’s got some nuances which are not intuitive (specifically with defence) so I think for that reason the Devs made the decision early on to not give us the numbers displayed in the game.
Assisted hit all is the same strength as the standard hit all when you have 1/4 of the element (just the user). It becomes double strength with 4/4. I believe it’s +30%, +60%, +100% but I need to double check that.
Ultra splash vs ultra wave I think it’s something like +25% stronger for dual target and +50% stronger for single target. As for all other moves like double survivor, double retribution, etc. it varies massively whether it’s stronger or weaker than the single target version (they’ve done it based on the moveset and stats of the monsters with these moves - typically they get designed based on the first monster to own the move then later ones get built knowing what the damage will be like).