Your time together with Crisis Core: Final Fantasy 7 Reunion will be relatively swift or purposefully glacial, and it all depends on whether you elect to engage in the game's myriad of side content. Zack's main quest isn't short, per se, but it's a blip on the ol' Shinra radar next to the sheer amount of optional missions available in this deceptively massive RPG.
The Sea Hulk, who players will encounter well on their way through that catalog of optional missions, serves as a famous stopgap between victory and surrender. It's lean, it's mean, and it's fierce. It's the ultimate behemoth-type enemy in Crisis Core, and we're here to ensure Zack Fair comes out on top for you.
Reaching Sea Hulk
Sea Hulk cannot be battled until players have reached Chapter Seven at the earliest. The reason for this is a little complicated since it ties in with not one, but two categories of side missions. You know how the mission guide lists things from one through ten, with digital designations such as 1-1-1 (the earliest) on through to 10-6-6 (the last)? Our attention must first tilt toward 3-2-2, which is to say the third category, second sub-heading, and second of six missions therein. 3-2-2 unlocks automatically at the start of Chapter Seven of the main game.
Beating 3-2-2, which is a cakewalk relative to what's to come, unlocks 10-3-1. The tenth and final category, third sub-heading, first in its little list. There are three here, so you need to do 10-3-1, 10-3-2, and lastly, 10-3-3. But your work isn't quite over yet - this, in turn, unlocks 10-4-1, which in turn grants access to 10-4-2, and - finally! - 10-4-3, the Sea Hulk battle itself.
Sea Hulk Versus Minerva?
First, a question for you, good readers. Have you already battled Minerva? Or gained access to the 9-6-X series of missions in general? It's technically possible to do so multiple chapters prior to when Sea Hulk becomes available. The majority of players will say that no, no they have not yet done so at this time. But if you've even managed to reach 9-6-1, let alone Minerva at the end of that trail (9-6-6), you are absolutely prepared to obliterate Sea Hulk and need no further guidance from us.
Consider the following. Minerva has ten million hit points on Normal and (wait for it) 77,777,777 HP on Hard. Contrast this with Sea Hulk's 52,800 on Normal and 425,765 on Hard. This is not to say that Sea Hulk is at all a cinch, but rather, it is a complete pushover if you've grinded your way to Crisis Core FF7 Reunion's true endgame fights.
If this isn't the case for you, and you're moving through Crisis Core's main questline at a more conventional pace, then please do read on!
How To Defeat Sea Hulk
Let's talk attacks. Sea Hulk primarily targets Zack with its claws and tail for purely physical damage. Gyro Tail in particular is pretty brutal, but it telegraphs by jumping back before swinging around in a circle. Avoid this!
You can't avoid Meteorite, which we've snapped a pic of above; like Zack's Limit Breaks and assorted enemy fare, it transports you to a separate little 'instanced' zone for a few seconds while it automatically pummels away for high damage. Last but not least, there's Ten Million Volts, which fires a blast of lightning-elemental damage across the beachside arena. It's possible to dodge this, which is good, since it'll stun you otherwise (unless you have the right setup to avoid such devious status inflictions).
Bring something super-protective along for the ride. The materia called Wall, which combines Barrier and MBarrier, works best, but Barrier alone will deal with most of Sea Hulk's strikes sufficiently when combined with well-timed blocking. The more you can outright dodge, the better, of course. But if you're a ninja at Crisis Core FF7 Reunion, you probably don't need our help. In fact, we may recruit you.
If you have the Twisted Headband, you'll be in a prime position to soak some of that damage up instead. Twisted Headband keeps Endure active. Endure prevents Zack from being bowled-over and stopped from using his own attacks when hit by an enemy's. It's seriously appealing stuff.
In terms of further Materia, we recommend Quake and Hell-FIraga/Thundaga, all of which can be used from a reasonably safe distance to keep the pressure on Sea Hulk. Equipment-wise, the best you've got is the best we can suggest, but if you've managed to snag ultimate content like Genji Gear, by all means, come prepared with it. Last but not least, Costly Punch is so good on so many levels that we'll just go ahead and link you straight to it.
Keep your defenses up and unload whenever possible, and it won't take long before Sea Hulk's a thing of the past.