Hello and welcome to my guide on how to earn the Ace of Sardine achievement.
This guide shall go over proper procedure on how to move / act when facing the dangers that lurk within the process of earning it.

Before we begin, let me state some important pieces of information:
-The achievement states "Defeat Super Synthesis Fish using sardine only". What this means is your party / squad can only consist of Sardines, not that you have to be a Sardine and the rest of the squad can be whatever.



-While there are methods to beat the Synthesis fish easily, they only really apply to when facing the boss with a standard Squad. Sardines are, as everyone knows, basically Glass Cannons / Cannon Fodder (They can dish out lots of damage if used right, but are also as fragile as a piece of seaweed). Basically what I'm saying is any method involving bum-rushing the fish or such are invalidated.
-Despite any discussions / chats about this achievement saying otherwise, this fight is actually very Luck based. Normally, you wouldnt think so, as your squad is ready to go with high level fish that can take a beating. However, when facing Synthesis with a squad that can easily die within a second, everything changes.
-To better explain the above, The boss randomly selects from one of its set attack patterns: Straight-on charge / ram, Broadside-styled onslaught, and fleeing to the other side of the arena to send in homing missiles / explosive fish. What attack it chooses to use is generally random chance. Also there is a chance that upon starting the fight, your Sardine squad will be stuck on the asteroid you spawn by, and be instantly wiped out by the bosses charge. However, this also applies to the boss itself, as it can also lodge itself into a nearby asteroid for a few seconds.

Before even thinking of engaging the boss, make sure all of your Sardine squad is maxed out in experience / growth.
Also it is generally best to only try this achievement AFTER you have already done everything else there is to do within the game.
Anyway, when you are ready: Change the squad to all Sardines, select the reef where the boss is found (as to not incur the wrath of the Giant Squid), and swim away a short distance / return back so you may actually enter the reef.

(Side note: I recommend switching control schemes to a controller for this, as you will need the extra camera / crosshair movement for this fight...or it helps atleast)

Once more I shall state that the fight can be very luck based at times, but with that out of the way, lets begin~

-Upon entering the arena, always make sure to switch your Squad command to something like Defend, as to have the Squad actually follow you around rather then aimlessly charging the boss.



-The best weapon / ability to use in this fight is the Cannon, as it does rather high concentrated damage if it hits.



-The main strategy is quite simple actually: When Synthesis gets close and initiates the broadside-like attack, Manually aim and unload on it with the Cannon.



When it retreats to the other side of the arena, ensure your squad is following you, then lock on to the boss and boost / strafe around him to ensure the homing fish shots don't hit you.
If it appears to be charging at you for a ram attack, boost to one side to see if it follows / leads your movement. If it leads you, quickly boost the opposite direction to dodge; if not, continue on.

-Expect your squad to be mostly all dead by the time you finally kill the boss, as the AI can sometimes slip up and/or get stuck on the asteroids.

All in all, the best way to describe one's experience when attempting to unlock this achievement: "Like fighting a Dark Souls boss".
It will take serveral tries, and most likely may cause moderate to severe headaches in rage; but in the end, it all comes down to Trial and Error.
You fight the boss, die, then re-enter; learning from the mistakes of the previous try.

And yes, a great deal of the rage comes from your Squad being OHKO'd right off the bat from a charge attack, or the boss spamming the 'Retreat n' Homing' method.

But by the end of it all, you shall walk away feeling accomplished.

[If anyone wishes to contribute pictures or such to the guide, message me.]