So you would need to move 6 units before you opponent moved one.
Unlike the PC game, lights don't move first, so after the first one lands, the warhammer could move away... say 3 hexes away from it, and light it up with PPC fire.
But assuming it stood there, the warhammer could kick and take off the spider's leg and most of the torso. In return, 6 spiders would give you 12 chances to hit. Assuming they jumped, and have pilot 5, and the warhammer is in open terrain and hasn't moved (best case scenario), that would be 5+3=8 for their TN, which is a 40% success rate. So out of 12 attempts, 5 would hit (approx). Of those 5 3-point hits, only 1/6 would hit the head, so 80% chance that they would hit the head once. It would be 15 points of damage total, spread around the mech. Not even enough to make for a PSR.
The warhammer kicks needing a 5+0 (he didn't move)+4 (max spider jump)-2 (kick) = 7, so 60% chance of hitting the spider with a kick, and crippling one.
Now if those spiders kicked... that would be 6 attempts needing 6's, which is an approx 75% success rate, so say 5 of them hit, doing 5 damage each, so 25 damage total, spread between the legs. This would force 6 PSR rolls (5 for kicks +1 for more than 20 points of damage, needing 6's. By odds, the warhammer will fail one of them and take more damage.