Maybe this is common knowledge but for those trying to solo the campaign on Insane, these guys can be quite challenging especially combined with the Serepedes at the end of Act 4, Chapter 5 (Bon Voyage).
The game itself doesn't make it very clear but here goes:
Armored Kantus can INITIALLY only be damaged by explosive weaponry. This includes Boomshots, Frag Grenades, and Torque Bows typically.
ONCE an explosive weapon of any type hits him, he will usually stop and roar with light glowing out of his mouth.
At this point, at least his head can be affected by ANY type of weapon. A longshot or torque blow blast will kill him instantly.
Alternatively, you can also unload half a clip of Lancer Ammo, etc and that will damage or drop him as well.
Area explosions can also eventually kill him but this will be from built up damage and works slower.
So the FASTEST way to kill the Armored Kantus on insane for me is to throw a frag grenade or Torque Shot him. When mouth opens, immediately torque bow him in the head and he dies.
On Bon Voyage when you have to overcome 4 waves of Armored Kantus, I killed them so quickly, that the second Serepede actually spawned with all the other enemies already dead. Those of you who have suffered through this part on solo Insane know that it isn't the Armored Kantus ALONE or the Serepedes ALONE that makes this section so frustrating it is the COMBINATION of the two that is challenging. Needless to say, only having to deal with a lone Serepede at that part was a huge relief.
Hope that helps clarify some things and makes taking down these guys a little more easy and straightforward.
The best way to deal with Serapedes on solo Insane is easy: don't shoot them. They attack whoever shot them last, so make sure that's not you. Also, shooting them in the head stuns them for a pretty long time, during which their tails are fair game.
All dressed up with nowhere to go... Walkin' with a dead man over my shoulder...
Nice to see some of the other strategies for dealing with him.
I didn't think to try incendiary weapons but could have sworn that on Seraphim17's Insane solo walkthrough on Act V Chapter 2 (Blackout) he managed to kill a Kantu with just a Scorcher. It was kinda chaotic there though so as has been stated here it could be the Incendiary stuff weakens him like an explosive.
I know about cheats but am assuming that we are doing this Solo Insane with no cheats.
On Bon Voyage, I agree that the first priority is to kill the Armored Kantus ASAP while leaving the Serepedes along. Still you need to be fast and accurate because the longer it goes on , the faster your team will be downed and when all 3 is downed, everyone will come after you which you can't handle . . .
Killing the Armored Kantu quickly makes it an even fight between your teammates and the regular Therons and Serepedes. You'll have time then to quickly finish them off while they are distracted and prepare for the next wave.