To quote my (work in progress) KD's How to beat each event (+videos) …
Enemy buff level:
Levels 1-60 = 0.7x - 1x
Levels 60-160 = 1x - 1.5x
Levels 160-300 = 1.5x (no change)
Levels 300-380 = 1.5x - 3x
Levels 380-400 = 3x - 4x
Levels 400-500 = 4x - 9.5x
Levels 500-600 = 9.5x - 17.25x
The buff should be fine up to lvl 300. All high damage strategies work. With the right SEs you should breeze through, or otherwise a good legendary will easily get you to 300. Level 300-400 is where most things stop working and you need to start using UC strategies or having crazy combos to win the battles.
However, with that said. 1.5x buff is no joke if you’re trying to get through it with normal attacks. You do need better things that just killer moves and such. Legendaries typically have far more useful attacks like blood moves that will help you get through it up to level 300.