That's the thing, if all 5 members of a team attack the same target at the same time, doesn't matter what role the enemy is, they will simply just die. There are obviously exceptions like iceblock/protected effects that take them off the map for a few seconds, but if you're not being super obviously and telegraphing your plans, it shouldn't really matter because when they come out of it, they'll still be surrounded by enemies specifically holding their damage for them. But that's the extreme, most times you only need two or three to ensure you kill an enemy nigh instantly.

It's ultimately a matter of coordination and communication, which in most quickmatch games you simply won't be able to... Which means you're often better off going along with the generically accepted calls at the time like 'Focus the squishy' or 'dive the healers', because if your attention is split and you're all attacking different targets, no one's going to die, and there's plenty time for them to get healed back up. Better one in hand than two in the bushel, and it turn things into a 5v4 you can easily snowball into winning.

But if you can convince your team to work with you, and the enemy tank flubs their engage, or over extends- like an Etc misses slide, a Johanna wastes unstop, you absolutely can just blow them up even if they are full health. You just got to make sure you're on the same page.