The landscape is your ally, GM
Placement of the players can mean a lot to the difficulty of the scenario. If the archers and spellcasters have taken safehaven in the back, have your reinforcements sneak up behind them and launch a charge to the weakest.
If your enemies are known as planar creatures or strong spellcasters, have the reinforcements "warp in" in tough spots or positions where they cut the player party in half, obscuring line of sight, and such.
Players often plan their strategies based on their enemies position. With a reinforcement correctly timed, you can be even STRONGER than you were if you had all the units placed from the start.
- As an escort quest, the players push their escortee backwards, towards squishy spellcasters and healers, while the strong-arms are dealing with trouble up front. Launch a second strike from behind or the sides, targeting their escortee, forcing the spellcasters to take physical defense.
- Create an encounter with bait. Some weak creatures are taunting the players, and characters with Charge or similar abilities rush to take them out. While this is happening, a swarm of burrowed creatures emerge from the ground, one or two adjacent to each player character.
- Thrid party combat. If the players are fighting some monsters to gain a treasure, maybe introduce a third party who are equally interested in the treasure, and will attack both monsters and players if they have to. But why should they fight, when they can just shoot some nasty spells at the players, and wait till they are out of the picture.. Or even better, buff the monsters ;)