Be a ranged.
Have any type of teleport/pos program. (I use Kefka bot to teleport, purchased at gg-bots.com)
Put yourself on the height just above Ravana's wings.
Tips: Ranava will aggro you and run directly under you. Ravana will not do any moves or attack. Because Ravana actually aggros and moves, anyone else who is in your party can attack Ravana for a faster kill, as long as they don't pull threat.
Other primals you can solo:
Garuda - teleport outside the area and kill.
Odin - teleport outside the area and kill.
Infirt - teleport outside the area and kill.
King Moogle Mog - Teleport underground (far enough to not get it, at least 7 yalms) and kill (works in ex, just kill the ranger 3x first)
Note: Works with most bosses that have an area you can stand on outside the arena, whose boss does not do ranged attacks.