The Problem isn't the aoe itself. It is the slow. If you do any kind of slow on the renaults, their own slow will disappear (which lasts very long). So they will faster go berserk through the party.
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Miasma II is 20 Potency initial hit with 10 potency/tick DoT. At 15s DoT time, with a 3 second timer, which is a globally timer. Even assuming you get Miasma II up right before the dot timer ticks, that's 14s left on the dot to tick. That's still only 4 additional ticks. So you get 20+10(x5 ticks) for 70 potency. That's a GCD wasted for 70 Potency. Ruin/Ruin II is 80 potency. You can use Ruin II and pop an off-GCD move for much better results.
Miasma II is totally useless in any single-target situation due to the lower potency. Sure you can contagion the Miasma II, and make it 28 Seconds (Again, assuming you're very fast on that Contagion after putting Miasma II up). That's 9 ticks for 90 Potency. So 110 Potency overall. 30 more Potency than your Ruins will be regularly. But you need to generally re-position yourself for this Miasma II to even hit.
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If you really want to have better DoTs up, and run up to the mob for no good reason, use a Potent Poisoning Potion -- That's about 20/tick dot and is affected by contagion, so you're looking at 9x20 Potency, or 180 Potency.
tl;dr Miasma II terrible for single-target. Any SMN thinking otherwise is terrible and shouldn't be in any hardcore progression groups.
do Tri-daster if you wanna bind it. it last 20 sec
Sounds like you dodged a bullet, OP.