I'll work on an explanation of where he is later, since I'm not good with directions in the swamp. Vendi is across the bridge, so I'd suggest equipping wellard for bonus health.
The battle has 3 phases. First is the tentacle sweep. Just jump over it to avoid damage. After a few swipes, it will smack down in the center, so just jump to avoid the shockwave. The eyes will now be open, so hit them! The small eye requires 1 hit to close, the middle needs 2, and the big one won't shut until hit 3 times. Next, a tentacle attacks from above. Just watch the shadow and jump when it stops to avoid the shockwave. Then, an eyehole opens for you to smack on a few times.
This repeats twice more with slight variation. Starting with the second go around, the area will be flooded, so your jumps aren't as responsive. Also, a new attack happens after the tentacle sweep. A tentacle will attack from the water, so just swim and jump in circles to avoid being caught. During the last round, the attacks from above and below will happen simultaneously.