Aether

Unless you're just running it to boost your arena score, Aether generally works best on units that can utilize Heavy Blade, Steady Breath, or a weapon that can consistently reduce their special cooldown. Typically these will be units that either have enough bulk to survive multiple rounds of combat or, in the case of heavy blade, enough offenses to reliably ORKO without a special.

Edit: Forgot to mention Dancing Blade, but apply the same rule as above for Heavy Blade.

Aether is generally good on people who have special charge +1 abilities like Heavy Blade, Dancing Blade or Steady Breath.
Otherwise, it is kinda meh due to long charge time, but does give you more arena points.

I run my Hector with Aether, Vantage, and Quickened Pulse. He's slow enough to get doubled, doubles with his weapon while baiting which typically kills them, charges up Aether and procs it on the second person to attack him. He runs this best because of DC and Vantage, but Quick Riposte and low speed are all you need, so many units can run it.

You kinda just send him in and let him do his own thing.

Wrath is another skill that works well with Aether, though to a lesser degree. Quickened Pulse on a unit that either doubles or gets doubled and drops them to half can have it at the ready by start of the next Player Phase. Because you want to KO the enemy, you'd generally want someone who is doubling but taking high damage, like a Life and Death Lyn.

I'm craving for Aether fodder now, I have two units in need of it :

* My Chrom supermerge (4* of course)
* Valter. I built him with Heavy Blade, he'll be so deadly once I'm done

Since the cooldown is so high, I would definitely go for a brave / stance or very speedy / QR user.
Dragons are good options (that typical whale build, massively foddering brave Ikes and Ikes to their dragons).

I wouldn't go with Lazlow, not speedy enough to double much or anything