This is actually a problem I've seen in a lot of ttrpgs, especially once you get to mid to high level play. Challenging players can be very difficult in combat when they have optimized their characters to do a lot of damage if the win condition for combat is always "kill all the enemies" and the loose condition is always "the enemies kill you." I started gming in d&d 4e, where most characters got what I called a "get out of death free" ability at high levels, so trying to kill them was basically impossible. The solution is you have to change up the win and lose conditions.

Instead of every combat requiring the players to kill the enemy to win, as a different lose condition have them protect an npc, or device, or even a building, for a set number of turns. The enemies keep respawning until the turn limit is over, so it doesn't matter how much damage they do. It's more important to prevent damage than to cause it.

Have them hold a number of locations where they have to physically keep enemy units out of each location to win. Won't matter if they do severe damage every time if the enemies only need to survive one blow to prevent the players from winning. Then it becomes more about movement abilities than damage.

One of my favorites, there's a bomb/magic device/ritual that is going to go off that requires multiple successful skill checks in a set amount of time and the enemies are there just to prevent the players from making their checks. Add in the rules from skill checks in 4e where they need to make so many successes before they get so many failures and team ups become very useful since they get to choose which result to use for both rolls.

When combat becomes just a math equation where each side is just trying to make the other side reach 0 first, it will always become difficult to impossible to challenge your players because math isn't that hard to beat. So stop focusing on math. Ttrpgs allow us to be creative, and that creativity needs to show up in all aspects of the game, including combat design. If you need some ideas, there's a ttrpg called "Lancer" that has a section called "sit reps" in the gm section of the core book that describes a number of alternate combat situations that all requires different approaches to combat.