The name of this level is probably a pun on "high terra", meaning "high ground". It's a fairly straightforward level, although you can get lost and confused at a couple of points if you don't know the layout.
At the start, switch to the cattle prod and just keep running right. Jump over the brown raptor and onto the vine, and jump over all the raptors after. Take care to move fast enough so they do not come up behind you and swarm you before you enter the cave.
You'll be on the mountainside. Move right and gun down the pterodactyls until you can't move further right. Jump up and start moving left; you should pass under a ledge directly below a pteranodon, which means you're going the right way. Keep moving left, jumping onto ledges as needed. Near the first large nest on that ledge, boulders will start falling; take care not to be hit by these. Jump up onto the ledges above it, moving left until you can jump to the top one and move right. Watch for crumbling ledge segments along the way, identifiable by the fact they are cracked and look different from the solid areas. Continue right, watching for boulders and two pteranodons. Prod the dilophosar near the cave and enter. Prod the compy and pterodactyls and move up and to the left until you reach a cave exit. Go through.
Three boulders will drop down almost as soon as you exit, be sure to avoid these. Go up and left, blasting apart the bluish rock with a charged shot until you reach a ledge just below one where a compy is running back and forth. Jump onto it, take out the compy and pterodactyl, and go right. Shortly, you will reach another cave entrance. Enter.
Ignore the first series of ledges. These lead you away from where you want to go and the stuff on the other side of its exit are not worth going after. Move left and gun down the compy and rock barrier. From there, keep moving up. A ledge within range of a dilophosaur can be a bit tricky. If you can, fire a charged cattle prod shot diagonally at it. You might be swarmed by five or so pterodactyls, so be careful. From the peak of that ledge, leap onto the next and continue up and left until you find an exit. Enter.
Above you, you should see grass and an iron support. From here, you will not encounter any dinosaurs, but you will face Biosyn. Move left and use a charged shot to blast apart the rock barrier. Switch to the machine gun (or shotgun if you prefer for taking out flamthrowers and grandiers). Keep moving left, shooting the Biosyn you see until you reach a ladder. Climb.
Diagonally gun down Biosyn above and take care of a grenadier as you move left. Diagonally take out another Biosyn gunner before you jump onto the rod and watch for another on the other side. Climb up and move right, shooting the respawned gunners. Ignore the ladder and keep moving right to pick up the medkit. Then return to the ladder and climb.
Take out the grenadiers from below before moving right. At the edge of the ledge before you jump onto another rod, shoot another grenadier and then proceed to cross. You'll probably take a hit from a gunner on the other side, but it is nothing to worry about as long as you shoot him when you land. Climb.
You should hear grenades exploding as you run left. Jump to see the grenadiers' positions and machine gun them from below. Don't bother with the gunner above the ladder where you came from, as he will just respawn later. Climb and take either ladder on this level.
Don't worry if you run out of machine gun ammo near this point. Just change to the rifle or shotgun (if you prefer it and it has any ammunition left). Shoot the gunners and climb up the ladder near the middle.
A grenadier will be tossing grenades near the top of the ladder. Jump off to the right and take him out before going back left and dealing with the running Biosyn. A flamethrower shout be aiming pointlessly at you, but ignore him and climb the ladder, going to the left and then shooting him from afar. Move right, take out the grenadier diagonally, and climb up another ladder.
Above you will be three gunners and a dish. Take them out, climb the ladder, and move to the dish. Congratulations, you're done!