Getting all 8 hits on a enemy God could be tricky, even if you have an ally (or allies) with a stun ability.
Luckily this can be obtained on jungle boss characters too. A jungle boss is like the Gold Fury/Fire Giant that can be found on the Conquest/Clash maps or the wild Siege Minion in the middle of the Siege map.
Simply walk up to any of these bosses and then just use your ultimate/4 in the spot they're standing and it should unlock after the last hit.