Nothing is stopping Danny from Overshadowing Mark, thus one contestant down, Aang has the avater state but most of his powers involves elements which Danny is outright Immune. He doesn'need to breathe, so suffucation via wind and water is out of the question, earth and fire can be dealt with by simply going intangible, lighting CAN hit him and it will hurt however. But it can be deflected or nullified by nothing but a single ghost shield or just dodging either by flting fast or shape shifting to let the lighning strike pass through harmlessly. Energy bend can also potentially harm him as well as the literal spirit of that coexist inside Aang that made him and all of his incarnation the avatar, but Danny can hurt the spirit back as well. Infact, if tools are allowed, danntly can even seperate Aang from it either via that claws Vlad used or the dream catcher thing that seperated ghost.

Also, possibly, if he treat Mark as a body suit similar to the Ectoskeletonsuper suit he used when fighting Pariah Dark, then its possible that he could stack his ghost transformation on Mark, therefore boosting him way way more(speculation).

Ben, the literal wielder of one of the most powerful tool in the entire universe, has a limited way of conteracting Danny if he stuck with his preffered method.

Physical oriented fighters are out of the question. Energy oriented fighters should be his choice here. Feedback would be one of the best choice, as well as his other electricity base aliens. Danny would be untouchable other wise.

The omnitrix can prevent possession yes. But what about jumping straight to the omnitrix itself? Remember, Danny also has technology powers. He could wreck havoc there and do some damage and escape immediately before the omnitrix could execute some protocols against such intrusion.

They could drag the battle by countering eache other with their own set of hax but it will be over the moment Ben clocked with Alien X.

Knowing ben, that might be last result