Level 20 cap solo

Carbuncle vs Leviathan Suppose to be kinda fair fight

but fight turned out to be very difficult why? beacuse he still use at low level Spring water that heals him almost full health.
In retail he did not use Spring water at level 20 fight never.

When it comes to fighting Leviathan and Garuda you have to play the waiting game and act quickly. Grind them down until they've used Astral Flow so the battle is safer, and then keep grinding them down to just around 25-30% HP and then pop Searing Light to hopefully finish them off.

I had the same problem and basically had to use this strategy. Astral flow right out the gate is always a case of luck (usually bad) and just having Carby grind them down is impossible because they heal over and over, so you need to snipe them at their weakest.