I know this is D&D and the default mode is to engage them in combat, but had you considered just paying the tribute?
Between a druid and a mage there should be very little you cannot supply.
Now, with the pressure taken off the town, you can proceed to utilise more sinister methods to ensure the army disperses and the ones who mustered it will have a difficult time in the future mustering new armies.
An army of that size needs supply lines. You, as a druid, have all of nature to act as scouts to locate those supply trains.
An army of that size needs food and water supplies stored in camp. You, as a druid, have infinite resources for despoiling or stealing those supplies. Vermin love grain. Skunks, and other musk spraying critters, can be sent in to render the supplies unpalatable. Venomous animals can be sent to poison food and water supplies. Suicide squirrels can be doused in plague or poison and sent to jump into the enemy water supply.
An army of that size needs a payroll! Find it. Raid it. A murder of crows can make short work of a pile of shiny coins.
Plagues of mosquitoes, fleas, leeches, and other biting vermin can make their lives miserable. The aforementioned skunks can make their tents uninhabitable.
Enemy spellcasters need 8 hours of rest to recover their spells. Deny them. Hit and run raids from squirrels, possums, small monkeys, ferrets, cats, dogs, and whatever can keep both the soldiers and the enemy spellcasters from getting any decent sleep. Exhausted enemies will make mistakes, and have a hard time recovering spells.
Use wildshape and Baleful Polymorph to pick off their scouts and sentries. Anyone who is left alone...disappears...
Send the rogue into the middle of the camp, Invisibility etc, Silence a tent and murder everyone inside it. Coup de gras should do it. Have him use a scroll of Animate Dead. Order newly raised zombies to wait until near dawn then leave the tent and attack anyone the encounter.
Pose as an itinerant holy man passing through on pilgrimage, offering a blessing in return for an evening meal and space by a fire...use the opportunity to sow a little paranoia with some tall stories about ghosts or curses, nothing specific or linkable to you or the town you are defending. Then move on. Return to bedeviling them.
Morale should suffer, and soldiers will start going awol. Let some go, have others suffer horrifically. Some die gruesomely to be found later, others survive to return to the army with tales of horror.
I doubt the spellcasters will be willing to fight to the death against an enemy they cannot even sense.
The Leadership feat makes note that spectacular failures and loss of soldiers in your command reduce the numbers of followers you can muster in future. This will count as that. And if you have raided their supply lines and encampments enough, you can run a profit, even if you did pay the tribute in the first place.
Just my thoughts...