Ugh, here comes Mr. Predictable. Heatran's Public Enemy Number One:
Blissey@ Leftovers
Natural Cure
Calm - 4HP/ 252 Def/ 252 Sp. Def
-Wish
-Protect
-Seismic Toss
-Toxic
A slightly different take on Blissey's more standard EV spread (252HP instead of Sp. Def and Bold> Calm). However, this is actually an under-rated spread that sacrifices some physical bulk to pump up it's special bulk. With these EVs, Fire Blast does between 43.3-51.1%, which won't be 2HKO'ing after Lefties - keep in mind that Bliss can Protect the first turn that it comes in, so it essentially gets two rounds of Leftovers recovery, thereby negating Stealth Rock as well.
From there, Blissey can just stall out with Wish/ Protect, and use Seismic Toss when Heatran misses with Fire Blast. The only concern is a potential critical hit, but you can't really take that into consideration here. Fire Blast only has a max PP of 8, and with Protect, Blissey can only be hit by 4 Fire blasts at maximum before 'Tran runs out.
Finally if/ When Heatran switches, it doesn't matter. Assuming Heatran has hit you down to 50% with Fire Blast, you can simply Wish the next turn as Heatran switches; follow that up with Protect, and Bliss is back at 100%, meaning that it can continue to come back in and wall Heatran, as it is completely unscathed.
PS: Note on the nature - I chose Calm to take Heatran's attacks better, though Bold is an option; however, Tran will do 47.4-55.8% damage, making it a bit more shaky in terms of countering this specific set. It's still possible with some Protect spamming and low damage rolls, but more unreliable. Also, if you want you can ditch the Defense EVs and drop them into HP, giving Blissey almost ridiculous Specially Defensive capabilities, though it's generally accepted competitively that Blissey really needs to invest in it's physical defense - but it is an option.
