The tank is there specifically to soak up damage and protect the others; if you can safely ignore it for a moment, taking down the harder-hitting assault characetrs or nearby support will make dropping the tank much easier and possibly leave you with dominance of the map. it This PSA brought to you by the three twits offed by Mercy in between bursts of her healing ray while they attempted to smash Roadhog.
It’s better never to let this happen, but iff you must go for the kill in this uncomfortable situation, shoot the weakest character first so that you at least knock someone down.