Inspired by the Amalgam comics character.

Dr. Doom's intellect/mind/soul whatever gets full control of HP Doomsdays body. He then covers that body in his typical armor, so he gets all the benefits of Dr. Doom added and stacked. He has full use of all his intellect, willpower, magic etc.

First round:
Khazan, normal conditions

Second round:
Dr. Doomsday has to find and kill Thanos when he is in one of his "farmer, wanderlust, hobo" moods. Meaning he isn't suspecting the attack. Doom gets full access to Latveria resources but the living tribunal says he's NOT allowed to mess with time, nor alter reality (prime mover) in regards to prep. He needs to find and kill Thanos in 6 months time. Achievable?