Mars vs Phoenix

How this matchup plays

Landing a perfect Arena of Blood on multiple heroes feels great, until you realize Phoenix was one of them. He doesn't respect the walls and can simply Icarus Dive out, leaving Mars looking foolish. Worse, he can use Supernova inside the arena, turning Mars's ultimate into a prison for his own team. All of Phoenix's damage is magical, which completely ignores the directional damage reduction from Bulwark. This makes it impossible for Mars to stand his ground and man-fight; he has to respect the constant threat of Fire Spirits and Sun Ray.

This dynamic defines the matchup from the start. Laning against Phoenix is miserable, as he can constantly harass Mars from a safe distance. Later in the game, Mars is forced to hesitate before using his ultimate, knowing it could easily backfire if Phoenix has Supernova ready. Mars has to rely entirely on his teammates being able to kill the egg, because he contributes very little to that effort himself. Phoenix, on the other hand, can play his game normally, knowing his kit naturally invalidates his opponent's biggest strengths.