Off the top of my head:

• Cast Fly yourselves or purchase/acquire some boots of flying
• Elemental resistance or immunity vs the element of dragon's breath you expect to be meeting, from the Protection From Energy spell or from equippable gear
• If you're confident in your spell save DC, Hold Monster is a fun one to cast on any flying creatures
• buy a bow and arrow
• better yet, buy a ballista and a mule to tow it around
• buy a couple dozen health potions that you chug furiously on the rounds that you aren't being breath-attack-strafed
• have the fighter make consecutive grapple checks to Los Tiburon the dragon into the fucking dirt

There are a lot of ways to deal with this, you just have to get creative and maybe set yourselves a short side quest or two to acquire what you need for the real job. Half of the above suggestions can be accomplished by a single tactically minded cleric, and no arcane caster worth his salt isn't going to know how to fly by level five, wizards that can't fly generally don't live very long and sorcerors that can't fly don't get laid.