Level 6 theory right here, basically, if you use flamethrower, at level 3 that's 120 damage per second in a cone, with a cd so low you can activate it again after its finished, plus the 30% buff from danger zone, which is about 40 extra damage. Add the ult, that's 150 damage or so instantly just by hitting, and 120 damage per second.
At level 6. With no items. Add some AP and a survivability boost from sunfire and some mr, you're basically an aoe dot MACHINE. Plus AP boosts your shield amount, and you can keep that up almost continuously. Add a rylai, you hit, they're boned.