Serra is very odd. I've played her in my BST-capped run (Camerupt, Medicham, Mawille, Magneton and Azumarill).
The best to do with her is to 1) outspeed her and 2) hit her with a move that has 100 accuracy or doesn't miss in the field. Her team on name is a crapshoot (Cryogonal leads, then Jynx, Aurorus, Vanilluxe, Glaceon and Frosslass). The problem is, 3 of them are fast, and her evasion boosts also boost the damage done by critical hits to surreal levels. Thing is, after the first boosted supercrit, the bonuses in both evasion and damage goes away until she gets her evasion boosted again (Double Team or switching, which she does a lot).
Strategy 1: Kill her lead, take the supercharged crit from her second. Die by the second's supercrit, revenge-kill the second and die by the third's supercrit, revenge-kill the third etc.
Strategy 2: Kill her lead, heal when the second comes out, to tank the supercrit. Heal again to mend the supercrit's damage and kill it. Repeat or mix it with strategy 1.
I'll see her in a new run with me at full strength, how she handles Close Combat Arcanine. Flash Cannon Blastoise and other good stuff.
And well, if the field goes away, it;s easier. Much easier. She needed Lapras and/or Cloyster.
There was a reason No Guard Machop got the axe from Pyrous. That wild vicious animal would destroy her.