I'd just play up the limitations of haste as much as possible.
Close range, so the party has to 'group for buffs' before combat starts. That puts them in fireball configuration and restricts what anyone does until the sorc's turn comes up. It also telegraphs to the opponents that a group buff is going to go off, so they can ready a dispel, slow, or counter or ready to disrupt the casting.
They only get an extra attack on a full attack, so space out the combatants so they can't get a full attack. Tumbling or spring attacking opponents are in order, as are reach weapons or trippers. Don't be afraid to use some disarm or sunder monkeys.
Short duration, try to stretch out combat.
Main thing is, if the party has a 6th level sorc, that's some magical firepower (even if it's not literally fire-power) regular peon warriors can't really compete. Now the peons need magic to level the playing field and they need to be tougher and be played smarter...which stands to reason, they are supposed to be harder, right? I wouldn't say that you need to raise the EL (much) but rather play with the encounter composition so the bad guys are more sustainable.
Compare 2 6th level fighters attacking 2 hill giants to a 6th level fighter and a 6th level sorcerer fighting those same 2 hill giants. In the first scenario it is a slug fest, who can last the longest, hit the most often, do the most damage... In the second, the sorc should be able to completely disable the hill giants while the fighter cuts them down. These are the same EL but obviously offer different challenge levels because of the relative groups' composition.
Don't nerf anything, don't houserule anything. If you can't figure it out after playing around with it for a while, start bumping up the EL until you kill the sorc
