There's not that many goblin and megaboss combos but mummies are a reasonably common enemy and their entire shtick is that they come in waves and start out weak but turn into tankbusters if you try to AOE them down from behind a multi-blocker. A setup where you're corralling them with a heavy and carefully thinning the herd out with single target DPS is already common operating procedure and your proposed buff would very good in that scenario. Honestly, I don't really intend that as purely criticism of your idea, however--counter-intuitive solutions to problems are often an interesting feature in games rather than just a bug. Your idea hit me as interesting and I basically just started thinking out loud about odd ways such an ability could be used. The only real issue I have with your idea is the part where it implies you're envisioning a future where heavies can tank multiple heavy hitters and I'm unsure how far in that direction the devs are willing to go. After all, heavies can already do that... up to a point.