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Worth: 30G

Description: Beat the game on any difficulty.

How to Earn: Quick tip - Use Psi Armor.

Eventually the story will direct you to build a Gollop Chamber. Once that's complete you'll be told you'll need a strongly psychically gifted soldier to advance to the final mission. (what you need is very high will; not psychic skill levels)

After you find a gifted solder through the Psi Lab you'll most likely need to put him in Psi Armor to get his will high enough to trigger the notification that you've got a soldier who's ready. (89 will, 20 of that from the armor bonus, was enough for my soldier). Send him to the Chamber to begin the end game.

Update:
At the end of the Temple ship is one final room to clear. The only entrance is a 1 wide doorway, which leads to short hallway to a ramp with ledges flanking it. D3dGr7sDntS4yNo pointed out that if you stay clear of the ramp area it's possible to preposition your squad inside the room before activating the enemies. That would be much easier than fighting through the choke point under fire. Once the enemies activate you get a cut scene followed by multiple Etherials and Muton Elites teleporting in.

Be prepared to deal with psionic mind control attacks (remember S.H.I.V.s are immune from those), I went with plenty of explosives; so consider alien grenades, blaster rockets, etc.

Update 2: It's only necessary to kill the Etherial who spawn on the central platform in the back of the room; you can ignore the other Etherials and Mutons who teleport in. I found a pair of leveled up plasma snipers with archangel armor does wonderfully. 2 or 3 plasma sniper rounds and the main Etherial dies triggering the final cutscene.