Specialy Defensive Heatran can deal with venu if its not the mixed set with eq(and that set has 4mss IMO). It sponges dual STAB and is immune to HP fire and resists Hp Ice and can use venus sun agianst it by firing of powerfull sun boosted(and hopefully flash fire boosted) fire attacks.
The Tran(Try over chandy)
Trait: Flash Fire
Moves: Fire Blast/ Toxic/Hidden Power of your choice /Earth Power
EVs: 252 SpD/ 200 HP / 56 SpA