This is pretty easy and can be done in about 20-30 minutes for two people. You can do this in a private lobby.
Mancubus have the most amount of health, which are the demons you are going to want to use for boosting.
You will also need to get to round four, where the two Demon players need to select Team Heal
as the end of round upgrade. Before getting to this round, each Demon player should take the health upgrade (Bigger Bar) as an end of round upgrade. The reason for this is because the Team Heal buffs as a percentage (50% health back to you, 25% to your teammate), so the higher your overall health capacity is, the faster this will be.
Co-ordinate between both Demon players who will be using the Team Heal buff first, as doing it too close together will result in a sub-optimal result (less healing if health is already high.) Get the Slayer to bring down the health on both Demon players to fairly low levels. Get the first Demon Player to activate the Team Heal. Get the Slayer to bring down the health of both of those players again, and then get the second Demon player to use Team Heal.
At this point the cooldown for Team Heal is 90 seconds, so during this time you can get the Slayer kill Demon players to help them grind
Both Demon players will earn about 18-25k health in a given 10 minute period, assuming the Team Heal buff is being maximized in a well co-ordinated way. This achievement will pop during a given match once a player reaches 50000.
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