Got it. Belakor has, on top of his T7 16W 4++:
-1 to be hit, cannot reroll hits vs him
-1 to be wounded
Access to a stratagem that nullifies one attack's damage
Single turn solution: Deathwatch melee. Reasons: because I am aware of it, and i don't know GK, who may be able to kill him with 28 mortal wounds or whatever, and AdMech also, maybe with lascannon saturation idk.>Method
1 chaplain, Single 5-man kill team armed with Heavy Thunder Hammers. Preferably in a transport so that they cannot he bewitched and can ferried quickly. That's it.>Wtf how?!
Wielding hammers means they already have a -1 to hit, which caps at -1. So they hit on 4+
Hammers are S8, so they would wound on 3+. -1 to wound, 4+. +1 to wound from Chaplain's Exhortation of Rage, 3+. +1 from access to "Get BA traits for one turn", wounds on 2+. Access to DW Specialisms (reroll 1s to wound vs certain Battle Roles): 2+ to wound, rerollable
Hitting on 4+, wounding on rerollable 2+, saving on 4+ invuln, four damage per unsaved hit, means 0.97222 per attack. I'd need more than 16 attacks to kill him
A Deathwatch Veteran squad can do 17 attacks on the charge. -1A from a nullified attack due to the stratagem, that's 15.5556 D inflicted on 16W Belakor, before the Chaplain yet swings. I think it costs more or less the same as Belakor's 280 pts. I'm not counting the price of the transport because it's also a fighter jet or tank, so that's its own unit.