As assasin / warblades are relatively harmless (and sniper when it's farm) compared to shamans / arcweavers, it makes no difference whether they die 10s after they spawn, or just right before the next wave spawn (heck, I know a lot of guilds on progression don't even bother with warblades and just let multi-dotters + tank to take care of them, resulting in >=2 warblades up at a time at some point)
3 seconds for both the shaman and Arcweaver, 1.5 for Assassin because of multidotting, total of 4.5 seconds tops.
Sniper is soloed by the disc priest(which I play), Ironblade is taunted to the boss and cleaved down.
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It is much safer to tank them instead of running away, because the chance they will hit you from behind and therefor kills you increases drastically then.
But again, talking about 10p.
Additionally, seeing you mentioned solo-ing sniper with your priest, I guess you were talking about 10 man. Shaman + Arcweaver + Assasin makes it ~25M hp. Killing 25M HP within 5s means your DPS (+1 tank) were pulling an average of 705k each during progression. Doubt that happened on every waves.