Sam, as a boxer you should be winning far more than you lose... Even if you decide to follow the headless chickens to do the blitz, and then find out that there is no tank... you have the firepower to down Vann. (just make sure that you don't pickup too much aggro from the bosses - let somebody else do more damage initially).

I won 3 or 4 games by tanking with 3x Voidwalkers (mage, priest & 3x locks), due to dithering team members ( i even soloed vann with 2 towers up,as we were running out of resources and needed a quick win)