If you wanted every synergy

(And actually have use of them)

You'd need:

Iron Maiden

Magic Sword

Dragon's Heart > Dragon's Magic

Excalibur

Cube > Akashic Record

Reaper's Scythe

Joker

Crown

Titan > Gaia

Opulence or Holy Chest

Nexus

And obviously you'd need MO.

That's pretty much the only setup that allows USE of every single synergy. As opposed to just having them and them doing nothing.

Countess alone won't trigger enhanced explosions. Maiden will. You'll also want a fusion or fusions that aren't CDR intensive.