He doesn't really need to teleport his head off, rapid teleporting his swords directly into the Hulks brain, heart and lungs. That would cause significant enough damage to kill or otherwise incapacitate Hulk. The worst thing Kurt could do is let the fight continue on. Time has always been the Hulks best friend. The longer a battle continues the better he generally does. And he only needs one attack to kill Nightcrawler.
If he's bloodlusted though he can just teleport his head off though. It was done in AoA, which isn't canon for 616, but there's nothing to suggest their powers acted differently. In 616 Rogue with NC's powers teleported part of Nimrod, and narration has stated Nightcrawler could if he wasn't averse to killing.