The Unspeakable Deep is found at the end of the tutorial mission on the boat and, much like the Vanguard in Demon's Souls, is a fight you're intended to lose the first time you attempt it. This is why it's recommended to not go after this trophy at the very start of the game even though it's technically possible to have this be the first trophy you unlock. Wait until you're more familiar with the game and its mechanics.

The Unspeakable Deep has the most easily avoidable attack out of any boss in the game. The problem lies within the fact that not only will your damage output be completely atrocious, but the Deep itself will also completely flatten you in two hits maximum. Since you won't have access to any good healing items, you will have to aim to defeat this boss without getting hit more than once. If you ever end up dying to it you will have to create an entirely new character to try again.

The Deep has two attacks: a jumping attack and another one where it smashes its hands into the ground. Regardless of which attack it's using, you should always stick close to its face and roll behind it whenever it starts up any of its attacks., then get 1-2 quick hits in. The best class for attempting this trophy is probably the paladin, as he has the most damaging weapon out of all the starting classes. Just remember to remove enough armor to be able to quickroll since your defense won't matter anyway and two-hand your mace.

This is a battle of patience more than anything, so the golden rule is not to get greedy. Manage your stamina so you'll always have enough to roll through it. If the Deep starts sitting in a corner, rolling through its attacks might be a little difficult so back off a bit to get it back into the middle when that happens. If you stick to these principles the Deep should go down sooner or later.

You may use this video as reference. Shoutout to Mah2c for this video.