Must-Have Items for Casters

[Lysander]

Web shouldn't a problem for a powerful wizard. Just have the wizard fly. Web can't be created without anchor points:

Web creates a many-layered mass of strong, sticky strands. These strands trap those caught in them. The strands are similar to spider webs but far larger and tougher. These masses must be anchored to two or more solid and diametrically opposed points or else the web collapses upon itself and disappears.

Have the wizard fly about 20 feet in the air. As long as they aren't flying between trees or something like that there won't be anything to attach the web to.

Or for a more direct approach Lesser Globe of Invulnerability should suppress any webs around them, and pretty much anything else your low level wizard can throw at them.

Just have the wizard use smart tactics. These spells will guarantee success:

Lesser Glove of Invulnerability (negates PC wizard)
Fly (negates PC melee attackers)
Protection from Arrows (negates PC ranged attackers)
Stoneskin + Mirror Image (just in case)

Note that all of these are buffs they can cast before the battle even begins. Don't worry about spell slots, all of these can be carried on scrolls. Then they can fly overhead and blast the PCs from above with destructive spells. Maybe even knock them out right away with some mass save-or-lose spell. You don't even need to give them a powerful magic item. Just give them a wand of Fireball or something so they don't run out of spells.