
Halfling Barbarian |

Well, at the very least I'd roll a perception check for them (if the spell weren't so noticable I'd suggest a spellcraft check). They may not notice things in the heat of battle. If the enemies are a cohesive unit with any sort of tactics communication it'd only take one to stop them. The PCs probably win either way here, since enemies are falling or stopping or moving slowly, so the wizard is still controlling the battlefield.

rangerjeff |
Given that Grease is the most commonly used 1st level control spell, pretty much all fighty types will have been told about it. Stories around the campfire, direct training. Heck, if they have any levels at all they've seen it up close and personal.
Now, these guys of normal intelligence just saw you cast grease. They watch you cast again. First thing they're going to do is look for the greasy spot. DC 10 perception check? In dim light maybe DC 15. And even if they don't see it, they'd get out ranged weapons and start shooting at the wizard while advancing slowly until they find the grease. If there's a highly logical spot for the grease to be, decrease the perception DC by 5.
I know it may seem mean to use tactics against players, but how dumb are bad guys supposed to be? And if a player complains that the bad guys aren't dumb enough, that player probably isn't playing for the adventure and challenge, but to stoke their own ego as they steamroll everything. If the bad guys see the grease in the bottleneck, have them tactically retreat and set up an ambush. After all 1st rule of why fighters are better than wizards is you can swing your sword all day long, while wizards only get so many chances to take you out. Make the wizard waste all his spells and then just walk up to him and slowly stick your sword in his belly.

lemeres |

Well, even if the opponents do not know anything about magic, I would imagine that if they heard him mumbling the phrase he used right before he made them fall on their bums would be a tip off. No matter what spell he was using, I would first assume it was grease again. Particularly since a bottleneck is such a great place for grease. (which would be an interesting tendency I would love to see exploited, but that is another matter entirely)
Also, grabbing butter, the material component, seems like a tip off as well. There is no reason for a man to grab butter on the battlefield that does not involve magic or cannibalism. Assuming that the enemies were not very, very tasty, then I will go with magic.

Lacan |

Also, grabbing butter, the material component, seems like a tip off as well. There is no reason for a man to grab butter on the battlefield that does not involve magic or cannibalism. Assuming that the enemies were not very, very tasty, then I will go with magic.
Grabbing butter. Lol. Now that would have required a perception check! If it was more than 20-30ft away.

Heaggles |
I play humanoids with int, so they see someone cast a spell and they fall down, then they get back up and they see the same person cast a spell, I would say that the leader would send the most expendable person or newest person to his group first to see if he falls down, and if he dose i would have the rest of them use range weapons.

Banalitybob |

I totally do stuff like this when I GM. My parties hate having to deal with the fact that their battlefield control tactics are outmaneuvered, or, even worse, matched. The whinging is just as bad as when I decided that using status conditions like sickened and dazzled on them was going to be fun. However, it's brought my group to a new level of combat where everything is hard, confusing, brain wracking, and totally awesome.
I totally use the whole "I use the same tactics and books you do" argument a lot. They gave in after I used the exact same Cavalier build one of them used in a previous game to wreck them on an open field. But yeah, enemies are people too, and a lot of times they have a higher intelligence score than half the party...