What you need before fighting The Silent One:
-All characters at level 15
-All extra stat boosts for your created character
-Maxed out attack items (x8 bottles, x4 clown bombs, x2 dynamite bundles)
-x5 strength weakening tonics
-All extra characters (T. Kemper, Anne, Fuschia) ready to attack.
-Final weapon upgrades for each character.
-A good understanding of counter attacks.
Here's how the boss battle will play out:
As soon as the fight starts, start tossing attack weapons with each character. The Silent One is weak to the bottles and the dynamite. Since he is constantly being attacked, he will be unable to attack you. After you are out of items, he'll have a decent amount of health taken away.
Now, have one character weaken his attack power and start pummeling him with normal attacks. Everytime you see the "attack down" symbol disappear, use another one. If it's taking to long to attack, use one of the support characters to keep him from attacking.
When he does attack you (and he will), know when to counterattack. To counterattack, you must press a shoulder button at the right moment, which is right when you see the enemies health bar flash white. Not only will the attack be dodged, but you will get a free attack against him.
The idea is to counterattack a lot and dish out regular attacks when you can. Counterattacking is crucial, though. The pace of the fight will go faster and you'll avoid damage (and losing your +attack stat from your super-powered weapons).
Also, using other items on your character (+speed, +def, +strength) will also help. You'll get to have better stats and the Silent One won't be able to attack.