(why is this a spoiler?) is really easy to use and a ton of fun. I basically made her my main as soon as she joined the party. Basically, you start the battle with an empty stack (last in, first out) of summons you have to fill before using. While a summon is in the stack, it has a passive effect on the entire party, which is cumulative (so if you have 2 thunder summons on your stack which raise the party's ether stat, that's better than having one). As soon as you unleash it, it leaves the stack and attacks, obviously (except if it has some other "unleashing" effect like water's healing).
As I said it's a stack, so the last element you summoned is the next one to be unleashed, with a max of 3 summons in your stack. I always summon 2 thunders right at the start of the battle (you get a "copy" art later which allows you to add a duplicate of your last summon to your stack without having to wait for the art bubble to refill so that's almost instantaneous), and then alternate between fire, wind and thunder to attack, keeping those 2 thunders to raise ether until it's obvious I can use them to kill whatever I'm fighting. Alternate those with Melia's other arts (two of which are used to topple) and she's pretty damn good at fighting.
If anyone has any other good Melia tips, please share!
also, what's the consensus on Water Summon? Is it worth keeping on Melia? I'm playing with Shulk and Dunban(sometimes Sharla or Riki, for affinity) as NPCs.