Panthalus is the gate to the World Tree, and it's the only fight on the route where killing the boss sends you all the way back to re-collecting the four Echobones. That single failure mode is what makes people dread it — so the whole trick is preparation, not reflexes.

Before you summon: prep an Electric answer (catchable as early as the desert), equip the Ring of Mercy so your damage physically cannot drop a target below 1 HP, and read the fight as a capture, not a kill. The encounter is scripted down to a sliver of HP where any sphere is a guaranteed catch. Fighting from a flying mount lets you reposition and keep pressure without eating the water attacks.

After the catch, the step everyone forgets: talk to the Researcher again. The catch alone doesn't close the mission — he does. Skip it and the World Tree chain stays stuck.