You should only use Miasma II if there are more than 2 enemies if you're below level 52 or 54, whenever you get Broil. After you get Broil, you only get damage gains for using Miasma II when there are 3+ mobs. It's also useful to put Heavy on targets instantly, as an example, the True Heart in A7, the flans from A3 or the orbs in Weeping City, etc.

The reasoning here is about the amount of total potency you get per GCD. Miasma II is worthless when used on a single target, because its potency is lower than Ruin, but if you use it on two targets, you're dealing a combined total potency of 140, whereas using Ruin you'd only get a total potency of 80. Its total potency only increases the more targets you hit. Basically, you won't be finding yourself using it too much in raids or trials, but it's good for trash.

I also noticed your comments about Place and Heel. Place is important at slightly higher levels of play, so you can set your fairy in a set location while you move to another one to deal with mechanics. A current example, in Nidhogg Extreme, you can place Eos to stay with the two tanks while you bait the fireball from the Shadow Dragon. Another thing I noticed is that you have Sustain in your bar. Sustain is pretty much pointless for SCH, since you can use that GCD on using Physick on your fairy which will probably restore more HP. That, and the fairy will heal herself anyways.