Pick a mainstream character then pick his nightmare matchup. The nightmare matchup character will a) be of a similar power tier, b) be unfamiliar to the first character [no rivals!] and c) pose a particular threat via their power set, experience level, mindset, or some other attribute. Offer an explanation for your choice. The outcome of the fight is irrelevant. For example:
Silver Surfer - Helspont
In addition to being super powerful, Helspont presents a level of versatility Surfer rarely encounters. Surfer would be at risk of being beaten at his own game.
To clarify, I'm not necessarily defining the nightmare matchup character as a instawin, just that he would be a very difficult match for the names character.
Not only would he frustrate Gladiator, he with his passive nature and psi could definitely cause Gladiator to question his mission and effect his power level.
Superman - Thor
A properly written Thor has a counter for Superman's heat ray vision, could effect the environment with storm powers and Superman would have to contend with multiple magic strikes as well as a fighting spirit to match or exceed his own.
Black Panther- Deathstroke
Deathstroke can match Panther in terms of prep, equipment and mental approach. I believe BP wins a majority, but he'd have to work very, very hard.
Wolverine - Deadpoole
Wolverine is up tight, Deadpoole is unaffected by anything. He has a healing factor to rival Logan and he's inpredictable.
He can become intangible. Has the ability to lift Flash off of the ground via TK (which would negate his speed) which leaves him with many options. He also controls time.