both skarm and forretress are 3hko'd by pert with wfall anyway, so what's the deal besides flygon?
The way i see it both skarm and forretress are tot not worth running if what you need is a ph wall, they lack a good recovery move.
Slowbro, arcanine and venusaur can recover quite easily instead and that makes taking them out quite difficult if your opponent has a good core.
The only reason why i like forretress is rapid spin, if it didn't have that i would have ditched it for an extra wallbreaker or a different spinner.
Edit: I'll run some calcs on waterfall vs skarm cause i never did before, i just 3hko'd one during a battle
252+ Atk Choice Band Swampert Waterfall vs. 252 HP / 252+ Def Skarmory: 58-69 (33.7 - 40.1%) -- 31.6% chance to 3HKO after Leftovers recovery
+ flinches
edit 2: Also yeah, low kick hera totally shits on forretress, same for any machamp.
+ yeah, firepunch never was a big problem to skarm, so i can see it being a better wall for that.