In case you can't get Hidden Power [Fire], and I don't know your current moveset, I'm going to list Mega Venusaurs most common moves (-HP Fire of course :P):
Giga Drain: STAB, passive recovery.
Leech Seed: Passive damage and recovery.
Sludge Bomb: STAB.
Synthesis: Reliable recovery.
Sleep Powder: Renders 1 opposing Pokemon inactive for 1-3 turns.
Earthquake: Surprise, Heatran!