Hmm... I'd say go with Physically Defensive on Trevenant, since Will-O-Wisp and a Physical Defensive set would help it wall Physical Pokemon. Here's what I suggest:
Trevenant @Sitrus Berry
EVs: 252 HP/ 176 Def/ 80 Spe
Standard SubSeed set, 80 Speed EVs over 76, because it out speeds 124 Speed EV Mega Mawile and opposing 76 Speed EV Trevenant. Here are other sets if you want to try some out.