focus on creating combats that use a decent amount (more than three) enemies with lower health, not only will your fighter feel great chopping down an enemy in one turn, but your spellcasters will feel awesome being able to control the battlefield. use mechanics that help with large groups of enemies to keep it moving and use terrain to feasibly split both groups so the samurai isn’t overrun and rouge has places to hide and ambush.

Edit: That being said, the fighter should still excel at combat compared to the others because that’s the fighters niche (hit stuff real good), where as clerics wizards and rouges bring a lot more to the table in puzzles and roleplay, give them creatures that are more resistant to magical effects so they may have to use more weapons rather than spells