We did it like this:
Every player can try two tasks per day. So they worked together...
Repair the Tollgate requires: 2 x Craft TN16. (Adds +1 Bonus die for all players for each of the 3 phases, if the gate holds)
Train Villagers: 1 x Insight TN14 (Draft the right persons),1 x Battle TN14 (Train them), 1 x Inspire TN14 (Get them to fight) (Adds +1 Bonus die for all players for each of the 3 phases if they survive)
Spy on the Enemy: 3 x Explore TN14 (Learn about each wave, per explore. Gain +1 bonus die for all players foe each of the 3 phases if successful)
I prepared some other tasks, like barricading the control room, trapping the riverbanks but the players chose to barricade the bridge and prepare fires on the opposite side... I granted bonus dies for that as well.
En route to Celduin bridge I prepared several encounters intended to delay the players:
They heard locals that had seen Lockmand with a huge chest and a few guards. That delayed the players when they went after him into the Long marches.
I placed an orc force, that they could ambush and prevent them from pillaging the countryside. They left them alone...
I had refugees from mirkwood try to cross Celduin, while being attached by GKs wolves. The players had to help or they would receive shadow for abandoning the refugees. They took the time to fight the wolves, but hurried on, without helping them build rafts...
In the end my players had one full day to prepare...
The battle itself was increased much in difficulty:
But I also gave the players extra options for their Hope.
1 Hope and Gerold would attack the strongest foe in your engagement
1 Hope and the Villagers would fire arrows at your engagement and make them weary for 1 round
1 Hope and Beoric, the guard captain in Celduin bridge would come to your aid with his great shield add +5 to parry for one round
1 Hope and you get a second wind. One of Erics daughters come running with a pint of beer smiling. Heal Basic Heart + 1d6 Endurance.
1st wave was led by Ghor, whom they had been defeated by previously. I gave him No quarter special ability, because he did not want to make the same mistake again, with him 2 Black Uruks and the wolfriders.
The players defeated him on the bridge and all his orcs. Total victory.
2nd wave was the Troll. A Hill Troll Champion. Favourite weapon and a great shield. 30 goblin archers.
The players killed the Troll before he even took one swing at the gate, our Lucky Hobbit did the job with his kings blade. They ignited the prepared fires (Oil and firewood) and charged the 30 goblins. They all Intimidated very well and routed everyone.
Grand assault: I made a list where for 12 rounds each player faced 1-3 opponents from a wounded goblin to an angry Hill-Troll. Every opponent slain granted the player a bonus die. Every round different opponents as the total chaos of GK's frienzied army charged.
After 12 rounds GK arrived, only a few could endure his presence, so now I used the original Feat die and rules from the book. I enhanced GK to be able to use Visions of Torment if the players failed the Fear test. That was pretty effective

In the end they managed to push GK in Celduin, heeding the Ravens last words.
It was an epic fight!!
I hope you can use some of this as inspiration.
Cheers