With a twist! This is not a battle of physical might. This is a battle of wit and words!
In the red corner, the Brit who won't quit, no matter how many nicotine patches you slap on him!
In the green corner, the God of Mischief, the lying Frost Giant who gives new meaning to the phrase 'cold-blooded'!
So what is the objective here? Loki has offended John's delicate sensibilities. John in turn, called Loki a right ****. For once, they both act reasonably and decide to settle this like gentlemen: Talk each other to death. They both must try to manipulate each other into either killing himself or getting himself killed. I.e., Loki cannot directly harm or cause harm to come to John, nor can John to Loki.
As said, neither can directly harm or cause harm to come to their opponent. So no knocking a construction site on their opponent, or hiring assassins to kill their opponent. Help also cannot break the listed rules.
Magic is allowed, but no magic that effects the mind or body of their opponent. That means no using magic to possess or control each other.
Each can call a meeting with their opponent at any point in time. Once a meeting is called, time is stopped, and each is teleported to limbo for the talk. When the talk finishes, each is dropped at the place and moment they were before the meeting. No magic or hostile actions can be used during this talk.
Keep in mind, while this battle is going on, the two of them, when not actively manipulating to win this duel, are going about their lives as usual. So Loki's plotting. John's hellblazing. Etc.
The objective is to manipulate each other until they die. So either manipulate the opponent into making a fatal mistake, or into doing something that results in getting themselves killed. They can manipulate those around the opponent to help.
Hope I didn't forget anything...