The Apex Barracuda is the first boss you will run into in Maneater. As you play through assorted missions in Dead Horse Lake, you will eventually be tasked with taking out the Apex Barracuda.
Where the fight takes place will be clearly marked by a waypoint, so there is no mystery there, but the Apex Barracuda is not an easy fight by any means. The boss will be level 10, so if you want to ensure you have the best chance at victory, it is a good idea to be that level, or even higher, before you take it on.
The main thing you need to know about the Apex Barracuda is that it is extremely fast, and definitely has the advantage over you when it comes to speed. The Apex Barracuda will dart it on and out, taking bites out of you as it does. This is where your dodge timing comes into play. Try to make sure your camera is on the Apex Barracuda as much as possible, so that you will see it dart in.
If you can time the dodge correctly, the Apex Barracuda will be exposed to a counterattack when it goes sailing past you, allowing you to get three of four bites in before you need to dodge out to avoid their attack. You can also use your tail whip to stun the creature when it is glowing orange, allowing you to easily do a lot of damage.
Try to fight near smaller fish that you can quickly eat to heal yourself, and avoid the large plume of blue water that is near the area. This water will burn you, making it an even tougher fight.