When you come out of a warp portal, and there is an enemy near by, get his attention to draw it closer. Keep in mind you have to still be standing in the middle of the portal. Let the dinosaur attack you until you health reaches zero. Be sure that you don't get knocked out of the portal or you will die. If done correctly you should be able to walk out of the portal invincible. Be careful because the trick will stop working when you pick up a health bonus.