Phase 1: Belly Shots
Kraid is a massive, stationary boss. In the first phase, he will spit projectiles and occasionally swipe with his claws. Your primary target is the glowing spot on his belly. Fire charged shots and missiles at it. When he roars, he'll launch a series of projectiles from his mouth. Dodge these by moving between them. After enough damage, he'll get angry and start launching lava blobs from his belly. These can be destroyed for health/missiles. Keep hitting the belly until he breaks the floor.
Phase 2: Grapple & Counter
In the second phase, Kraid will be partially submerged in lava. He'll launch more projectiles and occasionally throw a large, spiky ball. Destroy the spiky ball with missiles to reveal a grapple point. Grapple onto it and pull yourself towards Kraid's mouth. This will trigger a melee counter opportunity. Successfully countering will allow you to unleash a barrage of missiles into his mouth. Repeat this process, dodging his attacks and using the grapple point, until he is defeated.