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ok bring lost of high level bombs you know plus bombs than get Alatreon's hammer also bring the weaker bombs because there still good easy kill

will than get something with dragon because that dose most damage and stay to the head that the sweat spot on him soft like raw meat but more tasty

all rustshard weapons have dragon but hard to get best go with the Tough Break easier to get and its how i won but i never tried to kill it

* 2 Light Crystal
* 8 Pelagicite Ore
* 1 Jumbo Pearl

to get the first form than you can see how to up grade it to Tough Break red, blue, or yellow

smile

There is few ways of destroying Ceadeus, but by far one of the most easiest way of killing him is with a particular elemental DPS weapon that plays on his weakness. That can be either the Sword&Shield (SnS), the LongSword (LS), or more preferrably, the Lance. His weakness is weak vs. Dragon or Thunder.

What Majin Brandon suggested to you is partially correct as it's one of those weapons containing Dragon element, but it is flawed, ineffective, and impractical.
-First, you only have 30 minutes to either Repel or DESTROY him, and a SwitchAxe (SA though one of my favorite weapons) fails here because it's too slow. You'll most likely repel him.
-Second, the Sword Mode of the SwitchAxe is the only part of the weapon that has Dragon element due to its special phial, "Dragon Phial". You can actually DPS it for so long until your gauge bar runs out, which means you'll have to reload it again to use. That's probably 1-2minutes evading in between Ceadeus's attacks while you're loading your SA for Sword Mode.
-Third, it's not a DPS weapon, nor is it newbie friendly. The one combo of the SA I tend to see people use is the one where they often swing continuously, which eventually stops once the player runs out of stamina. It's weak, and most of the SA's attacks are weak except It's Overhead Swing, and Upswing move.

In Underwater Battles, Lance is overpowered. Sooner54, your choice of the Doris Spear (Y) + Lagia Armor (Elemental Up SKill)was actually a smart, efficient set for this boss battle. However,why you chose Doris Spear (Y) is beyond me when you could've chosen Doris Spear (R). More Raw Attack Power is always preferred over Affinity. Also Rathalos (LowRank BladeMaster) is better because of Attack Up (L).

What you wanna do with that set in the beginning is.....at the start of the mission. Grab everything in the Guild Chest except the bullets.
Part 1
-Eat the Rations. You always want full stamina when using weapons that includes a shield.
-Hop into the underwater channel and the moment you see Ceadeus, immediately lay your EZ Barrel Bombs and blow it up with either your Lance or small barrel bomb at its face. You do this very early in game because it's alot harder to keep him place at the 2nd phase of the battle.

Now here are breakable parts of Ceadeus: Horn, Beard, Tail, Back. If you want more rewards, you should should aim to destroy in the order Beard, Tail, Horn, Back.

-After the bombing, basically keep attacking, and resharpening your Lance to keep its Green Sharpness at the Beard. Until you see strands of hair after he whips back in pain, you're destroyed the beard. He will now swim faster to the end of the Waterway Tunnel.
-Resharpen your weapon and drink Dash Juice. You drink this because you will have unlimited stamina for a small duration of time. Meaning, whatever attacks you use will not consume stamina for that time, until it expires. The best move to attack a fast swimming Ceadeus is to catch up from behind with the Lance's Charge attack. With unlimited Stamina, you'll keep charging at the tail until your weapon breaks (which you'll have to resharpen again) or until the Tail breaks. Also you're damaging him alot earlier before the 2nd part of the match, so it saves you time + you get tail rewards. (To make Dash Juice
, combine Dash Extract and Catalyst together.)
-Pretty much you chase him all the way to his little spot at the end.

Part 2
-This part is usually funny. If you dealt enough damage to him, the moment you attack his head in part 2 of the battle, his Horns should break right off.
-Otherwise if you haven't dealt enough damage because your DorisSpear (Y) is low rank, well, keep attacking him. Eventually he'll rage. Signs of rage is Red. And as usual, he swims far away, which is where you swim to the Dragonator and spear him. Another timesaver and that should either break the Horn or his Back. If you did everything right and hsi Back breaks, you've broken everything. Goodjob, but usually High Rank weapons are able to do this.
-Pretty much keep spammming anywhere on the part of his body when you've taken care of all breakable parts on Ceadeus. Just remember to resharpen the weapons constantly, as damage is also based on Sharpness. Avoid his Blah Blah move and super Gay Water Blast and you're done ;o. Either Repel or Slay, who knows because you know, you're using a Low Rank Weapon.
-Of if you did kill it, great job. Fingers cross for Deep Dragon Gem ;o.

PS. Avoid using Ballistas in the battle. It takes too much time gathering, and while to shoot. That wastes time. Also learn how to counter Roars with the lance. If you can do that, you can attack.....FOREVER.

Possibly this little guideline should help you, and for some that needs Ceadeus.....VANQUISHED fastest.

Lawl. That's pretty much brief detail. If you can find a way of shortening it, and keeping all the important stuff intact, by all means do so.