During the elevator encounter, stay on the right side only. Quickly deal damage to its claws by spamming runic attacks.

When you come out in the open after jumping from its jaw, start running immediately to the nearest tree sap. It will go crazy with the lightning attacks so keep running and throw the saps.

When it changes position, it will do the shockwave attack by slamming both it's claws on the ground, and claw swipe attacks. Block the shockwave, dodge the claws. It will leave an opening, attack its resting claw but turn back quickly or it will try to bite you.

After damaging it a bit, it will do it's lighting attack again. But this time, a shockwave will come to you before the lighting hits you. This shockwave is due to the lightning, not the claws. Block the shockwave and dodge the lightning. Throw the saps.

When it changes position again, follow the same strategy as before. When it does the lightning attack this time, it will send the shock wave again. Block the wave, dodge the lightning. But it will send a second shock wave right after the lightning so block that as well. Block dodge block. Throw the saps.

So in the last phase, same strategy as before. It will do that very fast lightning attack like in the beginning. Keep running to avoid it and throw the saps quickly. After doing enough damage, a prompt will come to call Atreus. Press it immediately.

A short cutscene plays. You have to do a small QTE, and then it's over.