A combat is (almost) never only a combat.
If the players are confronting a bunch of monsters and nothing more, front to front, then the numbers speak, better numbers, better combat result.
The setting of the combat should bring some fun and tactics for the group with the upper hand (monsters or players), so try to include some extra stuff into play, some examples:
Ambush: If your players have a front and rear line, maybe the monsters could try to hide so some of them can attack the rear line (healers, casters). The first maneuver for a bunch of lesser creatures could be split the party so they can bring down healers/casters first while the others try to get into the combat to save his comrades.
A missing attack never vanishes into thin air: Where did that fireball hit? And the arrow? The barbarian swinging the axe? Maybe a column is hit and it crumbles splitting the party and blocking the healer's path and line of sight, maybe the fireball ignites the room, maybe the lost arrow activates a panel with a trap. This is specially useful when your players roll a natural 1, and since the fumble rules are non-existent get creative!
Furniture attacks: I think I saw on the D&D 4ed some rules to use scenery in some ways, like a brazier in the middle of the room, let's say that you can make a STR check to knock it over and make an area attack (DEX save, half damage and stuff). The lamp in the ceiling? Hell yeah! Let your ranger notice with a perception test that he can shoot the lamp and it will crush enemies behind (again DEX save, damage and maybe stunned or restrained on a failed save)
Weather and nature: Some classes like druids could get some advantage, bonus damage or higher DC saves for his attacks if the weather or the setting is appropiate. Maybe its raining, lets boost that Thunderwave!! An enemy walking on a puddle? Lightstrike his ass!!! He is now vulnerable and takes double damage. With some minor spells that can create water or obstacles you can get creative and boost other attacks, reward the creativity and show some!
Minions! Attack! Im right behind you!: Also on D&D 4ed there was a feature I liked a bit, the use of minions of 1hp (lets say 10 hp), his stats should be challenging in bonus attack and AC, but they go down on a single, maybe two hits. This is a good way of boosting your final evil magician mastermind, as he uses his reserves of infinite invocable minions to block line of sight, movement and making the PC's wasting their attacks on them.