Celebi may be superfluous with Ferrothorn, but whatever floats your boat. I can see how it trolls last 'mon Reuniclus or something.
If you find Tar not working for you, than consider a Mix Attacking varient. It can still set up Rocks and destroy certain defensive cores like Gliscor/Ferrothorn/Jellicent.
Heatran's typing is usually more useful in taking special attacks like Ice Beam and Draco Meteors. A Modest nature with 252HP / 56 SpA / 200 SpD spread can work.
I would be careful of Gengar though. It can outrun and 0~2HKO your entire team with the correct moves and with a Sub up, it can be particularly annoying to take down.