You expect your group of level 3 characters to be able to take down an EL 5 challenge in *two* hits (one for each guard)? Unrealistic expectations, to be sure.
A single casting of sleep has better chance of taking down both guards at once than attacking them. A DC of 14-15 should be expected, meaning the level 3 Fighters will both have to roll 12-13 to stay awake. The biggest difficulty is getting the spell to affect enough HD. Not a great chance, to be sure, but it's a chance to pull off a difficult feat.
Maybe another problem is that you know what the enemies' levels are. Why would your DM give you that information in the first place? That's ruining part of the fantasy itself, and encourages meta-gaming more than any rules-system ever could.
A single casting of sleep has better chance of taking down both guards at once than attacking them. A DC of 14-15 should be expected, meaning the level 3 Fighters will both have to roll 12-13 to stay awake. The biggest difficulty is getting the spell to affect enough HD. Not a great chance, to be sure, but it's a chance to pull off a difficult feat.
Maybe another problem is that you know what the enemies' levels are. Why would your DM give you that information in the first place? That's ruining part of the fantasy itself, and encourages meta-gaming more than any rules-system ever could.