While lightning strikes were capable of killing demonic beasts and even Champion demons, it should not have been enough to take down a High Champion Abyssal, but it did not need to.
The lightning strikes did not need to kill the High Champion Abyssal, only stop his passage long enough for the Abyssal Lord to no longer be able to support the portal that led from Abyssa to Terra.
Once you go through a portal that connects planes billions of kilometers away, you can't merely pull back if things go wrong. Once the Abyssal Lord's supernatural power no longer supported the portal, it faded, and the part of the High Champion Abyssal that was not out of it must have fused with the rock. Even a Guardian would have perished if half of their body was pulverized by rocks, much less a Champion.
