The first available time for this achievement is when you reach the Candy Kingdom. Once you enter the gates, go to the right and there will will be guard checkpoint with a level 5 banana guard along with 2 level 1 banana guards in the group of three. What I did was roam around the kingdom and smash all the destructible marshmallows, buns, and donuts to get money and health items. Roaming around the entire kingdom will allow for the destructibles and enemy groups time to replenish, therefore making this area a prime spot for farming to at least level 4 for Jake and Finn to fight the banana guard. Build up your characters to level 4, try to max the special attacks, regular attacks, and health bars when each level is achieved and the fight should relatively easy. If you don’t kill them on the first go, don’t worry. There is a benefit to dying as it will reset the entire kingdom’s destructibles and group fights to get more money and experience.
- Do not go to the Head of Security. This will pacify some of the groups to fight against making it harder to get experience. There is plenty around the kingdom to destroy and fight without continuing to the next part of the main quest.
- Build up the character’s special attack bar for the fight. Finn has Billie’s glove and Jake has his steam roller. This will help knock down the HP of the banana guard.
- Come with as many 50% heart potions as possible. This will helps keep the team members up and healthy as the level 5 will hit for a good amount of damage. The 50% is based on the character’s base total HP. The more points in the stat, the better the potion will work and heal for. These can receive these from destructibles or buy them from Choose Goose - leaving kingdom by boat and coming back again will refresh his stock.
- Bananas have a weakness to ice. Using the Ice Gem to change basic attacks to ice and Finn’s Ice Sword will do wonders.
- The groups you will fight in the kingdom with respawn on occasion as the character travels a farming path. Leaving the kingdom by boat and immediately redocking the boat will allow for a refresh, too. I did this BEFORE ever reaching the head of security.
I am sure you could get this naturally as the game progresses. It comes down to person preference - do you want to get this sooner or later - more for the acheievement hunters. This is the place I found is the earliest in the game to have the option to receive this achievement. Happy hunting!