okay well i'm not sure forretress is worth using because it is a terrible spinner without pursuit support. i suggest cloyster since surf does a good chunk to gengar and it can beat forretress, claydol, and skarmory, and it's immune to magnet pull. spikes are really important (they are required for msdos to be close to decent) and cloyster gives you a reliable spiker and self sufficient spinner that is pretty good against enemy stall teams and has a decently powerful explosion even without any investment to help take out walls. it also works well with metagross or jirachi (i was going to recommend jirachi before i saw how dire this forretress situation was...)
as for gengar, i think the specific set really depends on the rest of the team, but it's pretty much guaranteed that it deserves a spot.
Cloyster (M) @ Leftovers
Trait: Shell Armor
EVs: 252 HP / 80 SAtk / 176 SDef
Relaxed Nature (+Def, -Spd)
- Explosion
- Spikes
- Surf
- Rapid Spin
as for gengar, i think the specific set really depends on the rest of the team, but it's pretty much guaranteed that it deserves a spot.
Cloyster (M) @ Leftovers
Trait: Shell Armor
EVs: 252 HP / 80 SAtk / 176 SDef
Relaxed Nature (+Def, -Spd)
- Explosion
- Spikes
- Surf
- Rapid Spin