If you want to win, go singles against her
Doubles is sure troublesome
If you can get something to manipulate her field it would be of great help, freezing, corrupting or even summoning a terrain on top of it
You can counter her underwater field with your own diver
Freeze dry users dunk amaria really hard, so does shedinja w/ wonderguard
If ya getting outsped, trickroom can be useful, but it does not last enough
Electric attacks destroy amaria too
Try setting up X items on her lead starting by Xspdef and you should do fine with any mon
Vanilluxe might 1v6 her, pangoro too (last time i battled her, my xitem-boosted pangoro did all the work for me)
Ludicolo might prove useful too