Last night while rifting it forward, I joined a game where the portal % was very high (90% or above) but the guardian was not yet spawned. The RG was spawned, killed, then the rift closed and I was awarded the achievement.
1. Join game where rift % is high, but the guardian has yet to spawn.
2. Spawn rift guardian, kill it, and quickly turn in the quest.
3. Get achievement!