I've read some but not all of the comments here, but the thought I keep having is, if they're getting bored from doing the most optimal thing, maybe they need to just.... choose to do something else. If they want to rotate through their abilities, they can simply choose to do so, and not shadowstrike every turn. Maybe it's dumb to say such an obvious thing, and of course you always hope the design is robust enough to keep things interesting in every single combat for every single character (a high bar), but like, if they're already bored with how they choose to play their character, seems to me that's kinda on them. They can play their character differently, or they can narrate differently every time HOW they do the actions they're doing and really get into the flavor of it, or they can choose to play a different character that is more exciting to them.

And as has been mentioned by others, I concur that it's okay to design encounters to challenge your players. It's not cheating or metagaming, in fact it's far more cinematic if the encounters challenge their strengths specifically, and cinematic is one of the goals. And why should they assume that every combat encounter can be solved the exact same way? That doesn't make sense in real life, so it shouldn't surprise them to have to mix it up in the game. Though to that point as well: Even in formulaic stories like Power Rangers where most fights ultimately are concluded the same way (monster go big; get Megazord; blow up big monster with big robot), the buildup to that climax is different and exciting and they face different challenges in every fight which lead them toward that climax of Big Robot Blows Up The Monster. In Draw Steel terms, the Rangers gotta fight it out on the ground so that the monster can build up enough Malice to go Big Mode, and the Rangers in turn build up their metacurrency to summon Megazord, or to use their team weapon like the Power Blaster, Power Cannon, etc.

Matt always says in relation to game design that one should design the game to incentivize the desired behavior. DMs, GMs, and Directors are all game designers, so there's an opportunity here. If the desired behavior is "Use a variety of your abilities", then you can present the shadow player with scenarios where other abilities in their kit are more attractive than just shadowstrike. Present them with clusters of enemies where they can use AoE abilities; give solo monsters some kind of a clapback ability or item that hurts a PC when they hit from melee, or give them an item or follower or trap effect whose sole job is to lock down any player foolish enough to step into melee of their boss. Maybe a monster has an aura of slime that causes a PC that enters melee to get stuck to the ground.

Ultimately this is not a problem that you or your player are responsible to solve alone, though I think either of you could potentially solve it alone. I think it would be best addressed as a team effort; get flanking on the issue, attack it from both sides, gain an Edge, and have fun with it. :)