if i use moldbreaker excadrill, and serperior uses leafstorm on me, serperior's spatk will drop, due to moldbreaker.
If I have gyarados out,  and mega evolve on the same turn that serperior uses leafstorm,  serperior's spatk is raised, instead of dropping like it is meant to

It works as usual. Mold breaker affects the moves YOU use on a Pokemon with contrary, not the other way around. 

If a move used by a Pokémon with Mold Breaker, Teravolt, or Turboblaze would change a stat stage of a Pokémon with Contrary, the change will not be inverted.

For example, if you use Rock Tomb on a contrary Serp with a Mold Breaker Excadrill, the Serps speed will decrease as usual since Mold Breaker allows it to use Excas moves as usual. But Serps own moves' effects wont be affected in any way. 

Hello @SchmittyT9,

 

Like BlitzkriegZ said, Mold Breaker is an ability that works offensively, ignoring some abilities. It doesn't work defensively. Currently, this interaction works as intended in PRO.

 

However, every bug report is investigated and yours was very useful in finding an issue with Mold Breaker which I'll share so other users are aware of it: If you KO a Mold Breaker Pokemon with a Pokemon that has an ability affected by Mold Breaker (you can find the list of them here: https://bulbapedia.bulbagarden.net/wiki/Mold_Breaker_(Ability)), that ability will be suppressed the following turn and only that turn. So for example, if you KO a Mold Breaker Excadrill with a Contrary Serperior, on the next turn if you use Leaf Storm, the SpAtk will drop. If you click on the Serperior's profile it may sometimes show Ability None, which unfortunately isn't just visual for this specific turn. It goes back to normal after this turn though. 

 

This bug has just been reported to the Developers, there isn't an ETA for when it will be fixed. Thank you for your report and have a nice day!

Thanks all for clarification! 🙂