The Kaiser Behemoth is one of the optional enemies you can face in Final Fantasy 15 Episode Prompto's open world, but the only one that'll give you a Trophy or Achievement.
By taking on and beating the level 99 foe, you'll unlock An Emperor Deposed, so if you were going to tackle one side-quest in the DLC, make it this one.
If you're after help for parts of the DLC, consult our Final Fantasy 15 Episode Prompto DLC guide and walkthrough.
Kaiser Behemoth location
The Kaiser Behemoth side-quest is available as soon as you get the snowmobile as part of the Moving Forward main storyline.
The quest will appear from 15:00 in the north west part of the icy region, next to the lake, with a yellow marker signifying it's appearance. Otherwise, a smaller group of enemies will be in its place.
You can use the camp to the north (which featured in the Loss of Self main story mission) to skip time until it appears.
Kaiser Behemoth boss strategy
Though it's a level 99 creature, you can take it on at any time since Prompto doesn't level up as part of the DLC. That said, we'd highly recommend waiting until you complete Moving Forward and start Sins of Our Fathers, which sees Arenea join your party, making things much easier.
Kaiser Behemoth will be residing by the side of the lake, next to a hut. This hut contains several weapons to help with your battle, so be sure to keep visiting to stock up on ammo. However, don't stay in there too long - any attacks it'll do will travel through the walls, no doubt to stop you cheesing it from a distance, so you have to get more up close and personal.
The strategy we opted for is a very simple loop. Grab a Bazooka from the hut, run towards the Kaiser Behemoth and fire off a few shots at a distance.
Need more help? Our Final Fantasy 15 guide and walkthrough can provide tips on main story, as well as the open-world's many quests and activities. Learning how to get AP fast, EXP fast and money or Gil fast will aid you in many optional dungeons and tombs - including how to open and explore their locked doors. There's also plenty of interesting side-quests, too, such as the Scraps of Mystery and Professors Protege frog locations. And if you want to get around easier, you can rent a Chocobo, learn the infinite sprint trick and later in the game, get the flying car Regalia Type F. There's also more to see and do with DLC, such as Episode Gladiolus and Episode Prompto.
By which time, your Bullet Arts should be at around two segments (and if not, hang around until then) then activate Arenea's Highwind. Doing so will see Kaiser Behemoth enter a stunned state, which you can follow up by hammering the Triangle (or Y on Xbox) button to fire off a few Crackshots, both of which will deliver a chunk of damage.
Now, head back to the hut and grab another Bazooka, which should now have regenerated. By the time you use it, the Bullet Arts should be full enough to perform another Highwind - and so the cycle repeats.
There are three types of attacks to look out for while doing this; a lunge from above which you can dodge by waiting a moment before tapping Square (or X on Xbox) out the way, a projectile that's fired after the Kaiser Behemoth pauses momentarily, and close-up paw swipes and tail swings, which are avoided if you keep a safe middle distance.
The fight may take between five and ten minutes with this method, but it's simple and effective. Provided you've been good at hoovering up curatives as part of the main missions, you should be able to top up your health enough to easily last the distance and take it down.
Once beaten, you'll receive 10 CPU to upgrade your Snowmobile with, 5x Elixir and 2x Phoenix Down.