And don't forget, having a Warrior Mage come along to whup up on the party's Warrior Mage with the exact same spells and/or spell combinations...
But you would need to set it up...
1) Party runs into a group of bad guys as a minor encounter in a larger adventure, some escape (this is important).
2) Those that escape tell bigger bad guys, who hire specialists. Going from the descriptions of the survivors, they may start off by following the party, or bribing/paying/duping any hirelings into giving them info about the capabilities of the party.
3) Once they know what the party can do, they send in those that they think can whup on them. Including an enemy Warrior mage, who has time to prepare his spells before they ambush the party (i.e. the party Warrior Mage does not have time to prep HIS spells).
The best sort of back drop for this is a large city. Have the low level encounter be with members of the thieve's guild. They then consider the party a threat and the chief of the guild, being smart, is going to gather info before attacking and do his best to counter their special abilities. Like I mentioned up above, an ambush where their Warrior Mage has a chance to prepare while the party's doesn't. And if they know of other spell casters, they will also likely have a mage or two working things from the edges, casting dispels on various party members, trying to cancel out any active spells that they might have going on.
Hey! and being the thieve's guild, perhaps instead of trying to kill them, they decide to just beat the heck out of them and then rob them blind and leaving them without any of their equipment......