Vay Hek Terra Frame Problem

[Ograv]

Hi. After a few months away from Warframe I thought I'd go for Vay Hek. I farmed up a few beacons, crafted a key an in I went. I didn't have much trouble until Vay Hek reached Terra Frame in the final fight. He killed me once, I used a respawn, he regenerated some HP through a minion, but I got him low. Then... Then he became invulnerable.

 

I can't see anything on the wiki about him being invulnerable in his last stage. Am I missing anything? Is it a bug? He was sitting at 5-10% health or so, flying around, shooting me, sending missile barrages, spawning drones and minions, etc. I was shooting him and the minions that kept spawning for maybe 5 minutes with a potatoed and decently modded Dread and Stug, running out of ammo twice and dying another time or two until I gave up. He wasn't sucking up minions, his HP bar did not change (sitting still at 5-10%). I was soloing him so lag probably isn't it, though I did drop a lot in FPS at one point (after his invulnerability I think). So am I missing anything or should I post it as a bug?

 

Edit: My weapons did 0 damage.

Edited July 5, 2014 by Ograv

[xXHunterzzzXx]

Everywgere you shoot VayHek he is invurnable he is vurnable in head ot the place where that b***h sits

[KimchiKen_Prime]

Everywgere you shoot VayHek he is invurnable he is vurnable in head ot the place where that b***h sits

in terra frame you can shoot him anywhere, only in his flying mode u must shoot his head

[Ograv]

I was able to do it the second time. It seems the death scene didn't fire up for some reason. Posting it as a bug instead.