it doesnt resist a STAB flare blitz off of a 160 BP mega blaziken. How do you expect it to be a counter? Also if blaziken's already out attacking stuff, you cant even switch in your venasaur safely since it needs your turn to mega evolve..
3DS friend code: 3153 4218 0341 name: Kent ACNL town: Columbia ~Visit my Dream: 4800-2672-2882~
To counter a mega blaziken, the simplest option I see is Gale Wings (flying moves have priority) Talonflame. Resists Blaziken's STAB attacks, not immensely frail (could probably take a normal resisted hit well enough, but idk about sun/sword dance boosted hits), and has priority to counter speed boost. No need to waste a mega slot on it.
There are better counters around, but, ability aside, this is probably the simplest, and I've found plenty with the ability via wonder trade (secret abilities are breedable now, after all).
That way, venasaur can stick to only using think fat to tank non-stab low BP coverage moves, as it's meant to. Also, try special attacking giga drain (breed with a foongus/rosellia), sleep powder, sludge bomb (die fairies!), and synthesis-- Between sleep and healing, it's hard to take down. For physical, though, yeah, EQ and petal blizzard (or power whip, if you feel like egg move-ing it) will work nicely. Use a Sp. Atk or Sp Def boosting nature, maaaaybe a Def boosting one. Venasaur's high Sp. Def lets it switch into electric hits with ease, and EQ can take 'me out after doing so (it's not for switching into fire types, just hurting them on a predicted switch, or if they stay in while hit by a sleep powder on the switch.
Hope that helps-- if you need a gale wings fletchling or a Bulbasaur with giga drain or such, I can probably help out with that.
I wrote a guide to Monday Night Combat's Assassin, the threads were just sad: http://www.gamefaqs.com/xbox360/991788-monday-night-combat/faqs/60789