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Liquid Awesome
I don't think there is much point in even addressing this question toward the Cleric. Clerics do just fine in melee combat. Sure they don't have quite as much BAB and their Str might not be as high, but they should be wearing good armor and can easily wield a decent weapon (Heavy Mace). And so what if he has fewer hit points, he can heal himself any time he wants.
Druids also shouldn't have too many problems here. Their animal companions can help them out and, even though their weaponry is restricted, they've still got some decent options (Longspear springs to mind).
Sorcerers and Wizards are the ones that need to think more about what they are going to do when they run out of spells. First of all, I agree with those who say, "Delay until the Wizard casts Color Spray or Sleep, then jump in the battle." But there are lots of other cool spells they can use at low levels.
To address the specific situation of the Illusionist, Minor Image should be a spell that he casts in almost every combat (either by using Scrolls or preparing it multiple times). He can shape the battlefield heavily in favor of the party by using such spells. He can make opaque illusionary walls that enemy archers can't shoot through. He can make it appear as though a pool of lava or quicksand has been conjured near the party fighter so that the enemy melee types have to go around it to close the distance. There are many ways in which such spells can change the way a battle is fought to the benefit of the whole party.
As for what happens when he runs out of spells, he can fire a crossbow or use a sling. Will he hit much? Certainly less than a fighter would. But it is likely that the enemy will devote some resources to trying to counter his missile attacks and that is to the betterment of the party too.
But the main point to remember is this: If the party finds that after a couple of fights that the Cleric and Wizard are low on spells, they press on at their peril. Magic is the BIG EQUALIZER for the adventuring party. Most enemies you encounter will have the ability to melee but not all will have magical capabilities. Having your own magic as a means of countering what the enemy can do or making life difficult for their grunts (even if is the result of buffing your own party members) is likely going to be the difference between success and failure.
So ultimately the fighter types need to be more willing to take frequent breaks and thank their lucky stars that they've got such magical firepower on their side.
Druids also shouldn't have too many problems here. Their animal companions can help them out and, even though their weaponry is restricted, they've still got some decent options (Longspear springs to mind).
Sorcerers and Wizards are the ones that need to think more about what they are going to do when they run out of spells. First of all, I agree with those who say, "Delay until the Wizard casts Color Spray or Sleep, then jump in the battle." But there are lots of other cool spells they can use at low levels.
To address the specific situation of the Illusionist, Minor Image should be a spell that he casts in almost every combat (either by using Scrolls or preparing it multiple times). He can shape the battlefield heavily in favor of the party by using such spells. He can make opaque illusionary walls that enemy archers can't shoot through. He can make it appear as though a pool of lava or quicksand has been conjured near the party fighter so that the enemy melee types have to go around it to close the distance. There are many ways in which such spells can change the way a battle is fought to the benefit of the whole party.
As for what happens when he runs out of spells, he can fire a crossbow or use a sling. Will he hit much? Certainly less than a fighter would. But it is likely that the enemy will devote some resources to trying to counter his missile attacks and that is to the betterment of the party too.
But the main point to remember is this: If the party finds that after a couple of fights that the Cleric and Wizard are low on spells, they press on at their peril. Magic is the BIG EQUALIZER for the adventuring party. Most enemies you encounter will have the ability to melee but not all will have magical capabilities. Having your own magic as a means of countering what the enemy can do or making life difficult for their grunts (even if is the result of buffing your own party members) is likely going to be the difference between success and failure.
So ultimately the fighter types need to be more willing to take frequent breaks and thank their lucky stars that they've got such magical firepower on their side.