For me mega gardevoir was the hardest part, after resetiing like 3-5 times I was able to beat her with double sucker punch cinderace and bisharp which gets a field boost, Btw gilded arrows uses the secondary typing of the mon that uses it, if the mon doesnt have a secondary type it uses the mons first typing so Hydreigon has a +1 stab dragon type attack move with 80 base power
Edit: You can actually progress even if you lose to melia.