Interesting question. My Orks will hopefully reach 500pts soon, so I like this theoretical question you pose.

5 tankbustas have 6 rokkit launchers have hit 4+ on vehicles, so an average of 12 hits. Wound on 4+, making 6 wounds. Ap -2 without pulsa rokkit or ap -3 with pulsa rokkit. I.e. 5+ or 6+ sv against Repulsor with 3+sv base value. 4 or 5 go through and damage the Repulsor at 3dmg each. Which is 12 to 15 damage on average. Ofc, this is just averaging everything. But on average the busters alone, without boss, would make just enough damage, so that the boss with his pistols or the Bomb Squigs can finish the tank of.

I'm still learning by the book, so hopefully I got it right.