On map 2, find the small alcove at (H-9). The ??? is tucked away in the far end. Check it to spawn Dreadbug, which will drop 1-4 Giant Shell Bugs.
Trade the Giant Shell Bug to Ailbeche to initiate a cutscene in which he'll tell you where you can catch the "big one". You will still have the Giant Shell Bug after the cutscene.
If you have multiples and you are skilled at fishing up monsters, you can skip this step until after you fish up Odontotyrannus.
At this point if you're not experienced with fishing it's advised to look over the Fishing page to get an idea on how to know when a monster/NM has bitten the hook, and how to deplete its stamina when it does bite. Tip: When you get a bite, do NOT press enter/confirm before depleting all stamina or you will lose the catch and your bait. Do NOT press enter/confirm to let a catch go when you've hooked something other than the NM or you will lose your bait. Wait it out, and the catch will eventually escape and your bait will be preserved.
Obtain a fishing rod (any will do), then head to the small pond on map 2 of Castle Oztroja. Prepare for a fight, and bring a friend or two if necessary.
From the entrance, go through the Brass Door at (I-8) on map 1. To avoid the trapdoor, throw one lever and then quickly move backwards.
On map 3, use exit F at (G-7).
On map 7, use exit I at (I-7).
Head to (H-8/9) on map 2 to find a pool of water with Oozes in it.
Trade the Odontotyrannus to Zaldon (H-9) at the Fisherman's Guild in Selbina to receive the Knight's Boots. (Zaldon may not accept the fish while "Under the Sea" quest is active, finish "Under the Sea" to allow Zaldon to accept Odontotyrannus)