This is an interesting turn of advice I never considered. I have no problem cheating since the DM can't cheat I may well do that. In truth if it comes to that perhaps I am better at choosing another game. I will look into that too.
As a rule zero, I would advice not cheating. I would also advice staying believable and at least as much as possible avoid the appearance of metagaming. Set up scenarios where the enemy can be reasonably expected to have the resources to counter the PC's. Intelligent foes are best using generic defenses. The goal here isn't to beat your player. The goal is to make the game more fun for everyone involved, including the player you are challenging.
Summoned monsters are really abusable in D&D and in 3rd edition in particular, but they are also really easy to defend against. Attacking with summoned monsters tends to be one of the weaker things you can do with them, and stacking spells on a summoned monster which is only going to be around for a few rounds anyway is almost a waste of resources. Anything as simple as 'Protection from Good', a 1st level spell, complete negates most summoned monsters because they are unable to touch anyone so protected. There are also numerous ways to force summoned monster spells to end early. Look for anything that dispels magic or forces summoned monsters to return to their home plane. Make this part of your normal portfolios for spell castesr. So my first advice is to use monsters in teams with low level caster support. What you have is a spell user running amuck because he isn't being countered.
My second advice is that its hard to counter a capable party with a single foe. The party just too much dominates the action economy. My usual tactic here is to make the part face off against twice their number of foes, that are generally about 2 CR below party level. This makes for a combat that is hard to end quickly and generally avoids the beat down strategy that can occur when you allow PC's to concentrate all their firepower on a single target. Play with the enemy team composition and work on your tactical accumen. But never ever make up counters on the fly. Stick firmly to that believability rule, and remember that if you don't feel you can 'believably' counter the player then there is something wrong with your assumptions about what is 'realistic' because the PC can't possibly be the first character in the game world that discovered these sorts of tactics. In your world, countering enemy spellcasters is something every armed forces, every warrior cult, every templar of every church in the game has been training for reutinely. If the player can do it and does it reutinely, then you have to assume its Stardard Operating Procedure in the world and everything that couldnt' figure out some sort of effective counter is long sense extinct.
So let's make this concrete. Let's talk about some of the foes you'd like to include in the campaign world, and some of the standard attack modes the player uses with summoned creatures and how your world might have evolved to deal with them. Also, if this is a problem now, you may want to be looking ahead to spells like Planar Ally or Gate and thinking of preemptively rebalancing them before they get into play. The gated creature spells are much much worse than the summon monster type spells.