generally though unless you need to grab a neph stack, if the mix is bad we'll just lead them to one section of the map that are safe and either lose them off there or suicide, either way as long as the pack is out of the way, we'll just continue onwards if it's not possible to burst them one by one.
for invul minions though, you have 2 options as ranged...
1. is to use CC on them, stun, fear, or anything you have because they are only invulnerable to damage and not immune to the CC effect.
2. use attack that are penetrating, celestial orb, piercing orb, etc and what not (not penetrating by chance but naturally penetrating attack that will always penetrate, these attack simply coast through and will not be stopped by the invul minion) or wide area AoE blast.
incidentally, mortar sounds like an anti ranged mechanic but in practice since the mobs ALL can mortar you the end result is that you are going to get smashed either as a melee or ranged if u can't dodge it, for ranged dodging it is straightforward (though not exactly always feasible depending on the map), for melee however what typically happens is that the minions who are spaced apart from each other are fully capable of mortaring you regardless of which minion you target or the boss itself.
they effectively cover each other with overlapping field of fire and if you have to engage them in melee range, the only thing that can't hit you with the mortar is the one you engage directly, while his minion or buddies rain shell on top of your head.