Just before the basilisk uses it's ranged poison attack it will coil up. You can see the green reflection on your character from incoming poison attacks letting you know to change directions when running away. It's best to be on a fast mount because a hit from the venom ball will do direct damage to players even on mounts.
This thing is a titanaboa on steroids, if you see it’s tail popping out of the ground, try not to get near it unless you have a few strong fighters. If the gas ball hits your tame, get out of the cloud of poison it will probably kill you, except if you have a hazmat suit on.
If you are on foot, ignore them. They have good HP and spit poison; If you plan to take it down with a mount, use Hazmat Suit Hats or be ready to pump yourself Medical Brews, also backpedal to minimize the damage done to your headgear.
Apparently only a non encumbered raptors can outrun these beasts...and only if you run towards some other dino while running away so he attacks that dino while you are running away, otherwise he has same speed as raptors, 140% ms on human can outrun him as well...fighting without powerfull big dinos not advised, the poison kills you and basilisk will knock out other dino before you can do anything