See Full Series here: https://www.youtube.com/playlist?list=PLezAEu8shuO6qdBP5Qq_wjqRpDHhtOSDT

This is a Gym Leader episode where Metro describes exactly how he beats Angie.
If you want to see the attempt where he wins, just watch the video noob.

I recognize this fight has changed a lot in recent versions, but I'll still explain my logic here.

At the time the field could be manipulated in such a way that you could turn it from an OP ice field to an OP fire field. This creates a 100 percent win condition if you can survive a turn and NOT one shot the Abomasnow but get him into potion range.

Lava Plume has to be the one, eruption does not break it down the same way (at the time of recording.)

Try to go to fighting types for the water/ice and rock/ice or a strong electric type.