I was under the impression that her Transmutation could be dodged, but considering it's something that she would save for when in trouble that means Mash is safe until he can land a hit or Twilight fails to hit him a few times.
Mash avoiding detection here is debatable, so for ease sake i will say they just notice eachother at the same time.
Twilight uses anything other than transmutation: Mash avoids through IR or swats it away, then proceeds to either throw a rock or big bang dash on her. Big bang dash is blitzworthy, so even a shield is likely not saving her from a direct hit to the head, wich at 5.5x AP advantage (assuming her shield is equal to her AP in defense.) and Mash's skill level should likely be enough to K.O.
Rock throwing can probably be dodged by teleportation, but if she does get hit it would count as piercing damage since Mash can apply enough spin on his throws to make them behave like a drill, wich can be lethal at 5.5x AP advantage if it hits a vital spot.
If Twilight survives the initial attacks Mash will have trouble catching her even with her mobility and from there it depends on him being able to kill her before she decides to use transmutation, wich seems to lean in Twilight's favor, but Mash's IR can tell him when someone is about to use hax on him, wich could lead to a Big Bang Dash into the sky depending on how far above she is.
I'm thinking of an incon, but leaning more towards Mash since his instincts are OP.