Immediately take out the vandals around the edges of the map - them sniping you is unwelcome danger. Spread out, but not too far in case revives are required. There are many areas on the map where you can duck in and out of cover to avoid his very powerful cannon (obviously telegraphed with red laser).

All 3 members should have equipped (most likely switch to) the highest level + best impact sniper rifle as secondary, before picking up secondary ammo left by the previous stage. Focus sniper fire on one leg - call it out, "front left" etc. A couple of rounds from each of you, his armor will blow off and he'll go into a super vulnerable position for 5-10 seconds - shoot the red core exposed by his neck, again with sniper rifles. reload as soon as you see leg armor blow up. Two or three of this and he's done for.

Everyone should be aware of additional villains spawning, and take them out quick (scout rifle / hand cannon good for this at distance, shame to waste sniper ammo)

If you have them, rocket launchers to the legs are good too, but ammo for those is scarce, and reloading is slow. they are poor for shooting the exposed weakpoint in the neck.

Team members should carry a green ammo resupply for their rifles, available from the weapons merchant. Not required, but it certainly goes a lot slower if you run out of sniper ammo.

There are undoubtedly many other effective techniques too - this is just what I do and find quick.

Then on to Sepkis Prime - snipers aren't too bad again, I prefer to just hammer him in the eye with a high-stability autorifle + rockets when I get them.