Honestly, I’ve found that the best defense for Behemoth (at stage 1) is a good offense. This means taking the damage perk, putting at least 2 (if not all three) points into either Fissure or Lava Bomb, and overwhelming/intimidating the hunters so they’ll back off (or so you can finish them off).
I’ve been running with 3 points in Lava Bomb, and it’s been working fairly well. However, as Behemoth where you fight is just as important (if not even more important) than what powers you select. Learn to pick where you fight, not get caught running from a fight. Let the Hunters dome you at stage 1 when you’re full armor and you have a perfect spot to fight them in (preferably caves or places where the hunters can’t move around much; aggressive wildlife you haven’t already angered is a plus). Lava Bomb and Fissure do their best work here, and the hunters have issues dodging your powerful attacks. Smart hunters will often know that you’ll overwhelm them in that situation, and either drop the dome early or not dome you at all.
Sorry if I went off on bit of a tangent. I too have been trying to get better as Behemoth, so I thought I’d share what I’ve learned. I’d recommend 3 points in Lava Bomb and the damage perk at Stage 1. (Lava Bomb is a fantastic ability; it just takes some getting used to.)
At Stage 2 is where I like to grab my utility, so I go for 2 points in Rock Wall and 1 point in Tongue Grab. I find that 3 points in Lava Bomb + the damage perk is plenty of damage for me to take down hunters, so utility is what I’m really missing before Stage 2. 1 point in Tongue Grab is all you need to separate one hunter from the rest at close range, and you can keep that hunter separated from the group with Rock Wall. Feel free to go to town on them and down them after they’ve been separated. Lava Bomb, Tongue Grab, and Rock Wall are all fantastic abilities for camping a body relatively safely / at a distance, which make it that much harder for the hunters once you do take someone down.
At Stage 3, I usually choose 1 of 2 builds. Both builds have me take 1 more point in Tongue Grab. With 2 points, it has decent range, which is phenomenal for fights near the Relay (where the hunters try to force you to fight on their terms). The remaining two points I spend as either 2 Fissure (for extra damage and mine/trap clearance) or 1 Fissure 1 Rock Wall (for even better separation + mine/trap clearance).
Lava Bomb is good ability; don’t give up on it.
Choosing where you fight (are domed) is very, very important as Behemoth!
Stage 1: Damage Perk, 3 Lava Bomb
Stage 2: 3 Lava Bomb, 2 Rock Wall, 1 Tongue Grab
Stage 3: 3 Lava Bomb, 2 Rock Wall, 2 Tongue Grab, 2 Fissure OR 3 Lava Bomb, 3 Rock Wall, 2 Tongue Grab, 1 Fissure