Hmm... let's see...
Terrakion... I'm not entirely sure if Choice Scarf is THAT great. I've faced one in Wifi random matchups and I smashed it quite easily. Choice Band might be a better option, or even Focus Sash. But seeing how your Gengar (I think it's perfect, by the way) already has Focus Sash...
Slowbro... I've never, ever considered in using one before, so I have no comments in the using-perspective. I've faced plenty and one-hit KOed them all with Choice Band Outrage so I can say that my opinion on it isn't all that great.
Ferrothorn... wouldn't Sassy nature (+Sp. Def, -Spd) be better suited than Relaxed? With Curse, its Def is going to get really high. You MIGHT consider in having maximum Sp. Def.
Infernape... personally, I'd use Earthquake over Stone Edge because I absolutely hate Stone Edge's accuracy. You might also want Thunderpunch for Water types, but I'm not entirely sure where it'll fit. Keep Mach Punch as you'll need a Priority Move.
Galvantula... I don't see anything wrong with it. I know you should never underestimate the power of STAB Thunder that doesn't really miss.
135: "You play wifi random matchups?"
Me: "Being undefeated is fun."
135: "That's nice, but how many were against those who actually knew what they were doing?"
Me: "More than you'd think. I face one noob player per every two pros. But the point is, I'm undefeated against other RNGers."
135: "Perhaps. But the real fun comes from facing other pros in full scale 6 v 6 with no restrictions. That's where everything goes."
Moral of story: Random matchups can be fun and interesting, but a full scale 6 vs. 6 can be an epic clash where both sides bring everything they have into the field, including gimmicks, dedicated counters, and emergency responses.