Start by summoning Carbuncle.

1. Press Aethercharge / Deadwyrm Trance / Summon Bahamut / Summon Phoenix. It's the same ability but will have one of those names depending on your level.

2. Ruin will turn into a new attack with no cast time, imbued with the attack of that summon. Just spam that 1 button and you're good.

3. Enkindle Bahamut (turns into Enkindle Phoenix when Phoenix is active). Press this when available too.

4. Astral Flow turns into Deathflare or Rekindle. Press this when available too.

5. After Bahamut or Phoenix runs out, 3 abilities become available to you.
Summon Ruby / Summon Ifrit / Summon Ifrit II
Summon Topaz / Summon Titan / Summon Titan II
Summon Emerald / Summon Garuda / Summon Garuda II
You can press them in any order, so choose one.

6. After choosing one, Gemshine becomes imbued with the element of that summon. Spam that 1 button.

7. After choosing one, Astral Flow, which you used earlier, gets imbued with a special attack from that summon. Use that too.

8. Cycle through the other 2 summons, then repeat from the beginning.

There is an aoe version of all these abilities (Tri-disaster and Precious Brilliance) and there is Energy Drain into Fester and Ruin IV, but you can ignore these at first until you grasp the above things. It's a braindead job that just cycles through each primal spamming 1 or 2 buttons that are imbued with an element, mostly.

But I do find it fun.