Breloom has a new Dream World ability, technician, that gives Mach Punch a slight boost.
If you're still worried about Serperior, try this:
Iron Tail (egg move. Breed with something from 4th gen)
Spam coil after using Leech Seed. This saved my team late in the game vs. Iris and her dragons. It may not work for every situation, but physical attacks won't hurt as much outside of Brave Bird, and Iron Tail gives Ice Types something to think about. It absolutely won't miss after a few turns of coil.