So which old content i can do unsync?

[Astrahime]

I can do the hardest raids solo if I have enough DPS and game mechanics allow it.
So why i can't do the ultimate raids unsynced.

From the wiki on The Weapon's Refrain (Ultimate):

The boss will begin with four large pillars dropping at the cardinals of the arena and dealing proximity damage. Players should stack in the center to shield and heal through this. The boss will summon 6 Magitek Bits around the arena. They will begin slow-casting Self Destruct which will wipe the party. These can only be defeated in time by using the level three caster Limit Break. This should only be used by the caster with the Beyond Limits buff. After using this, the Limit Gauge will refill. Players should also be attacking the bits to ensure the Limit Break kills them all. After they are destroyed, Lahabrea will spawn and immediately cast Blight, which will reduce all players HP to 1, inflict Doom which lasts for 8 seconds, and Down for the Count, which lasts for 4 seconds. The only way to survive this is by using the healer level 3 Limit Break. Again, make sure the healer with Beyond Limits uses the Limit Break. The boss will become targetable and begin casting Dark IV, which will hit for max damage. Use the level 3 melee Limit Break to defeat him while other players attack the boss to ensure it dies. After Lahabrea is defeated, The Ultima Weapon will spawn and cast Ultima, an attack that must be mitigated via shields, cooldowns, and a level 3 tank Limit Break.

Please tell me how you're going to get through all of this without a full party.

Limit Breaks are only usable in a party environment. The entire party shares the same limit break gauge, with each party in an Alliance raid having its own meter. There are 3 levels of limit breaks (commonly abbreviated LB1, LB2, and LB3): the 1st level can be used in a party with 4 or more members, the 2nd level needs a full party of 8 members, with an additional level being granted while fighting a boss.

So you don't have ANY limit breaks accessible to you, and even if you did you wouldn't be able to use multiple different types.