As you’re discussing timered spells, I completely agree with using a timered spell and a spell you can spam. Also a good idea to choose a skill that doesn’t need synergies, and even low mastery if it’s a cold spell, and pair it with a spam skill that has strong synergies, lightning or Fireball.
Now regarding Meteor Orb, it has a timer of only 1 second. That makes it difficult to spam another spell in between. Blizzard for example haas a timer of 1.7 seconds, more time to spam another spell.
A thing I have been trying to make work, is Meteor Orb with Hydras. Idea is of course, that Hydra stays up for 10 seconds, and you are shooting Orbs meanwhile. Hydra has 2 second cooldown so you put them up front when you advance, and cast Hydras when you reposition and move around, and you can spam your Orb with aid from a couple of Hydras.
Getting 4 Hydras down which is max, happens with Cold Immunes, and they help your Merc so it doesn’t take forever.