Trevenant + Heatran + Rotom-W is a very good FWG core on a stall team that I have been using, and it is very successful; basically, these pokemon cover all of each other's weaknesses extremely efficiently. Heatran and rotom-w cover Talonflame, which trevenant can obviously cannot handle. Trevenant handles the odd Celebi that switches in on rotom-w, by setting up a substitute on it and 2HKOing with shadow claw, its substitute intact in the process. It can then set up Leech Seed on the pokemon that comes in after celebi. Both trevanant and heatran can handle any bulky grass types that dare stand in rotom's way. Trevenant and rotom-w can handle ground types, which heatran gets raped by. Trevenant and rotom both resist water, which heatran is weak to. Trevenant is immune to fighting, which heatran is weak to, and rotom can set up will-o-wisp on most fighting types (aside from Blaziken, certain guts pokemon, and the irrelevant Infernape), and can kill blaziken, a pokemon that both trev and heatran have trouble with. The rotom's set is physically defensive with volt switch, hydro pump, rest, and will-o-wisp. This is a chestorest set that is quite annoying for stall that relies on Toxic to wear down rotom. Heatran is standard, with roar/lavaplume/protect/stealth rock. Not much to say here, other than that it contributes a lot to this core, but I've already aforementioned its uses. Trevenant is a fantastically underrated subseed set with substitute, leech seed, shadow claw, and will-o-wisp. Trevenant walls a lot of pokemon, and can set up substitute on a lot of things. All in all, this core is very successful and very good on stall, in my experience.
Using a Gourgiest(huge) would be better suited than Trevenant to this role, it has similar special bulk to Trevenant, but it can sponge physical hits much better than Trevenant can, then proceed to cripple physical attackers with WoW, it also has a better support movepool than trevenant.