While every character has a tiny bit of synergy, the assassin is far and away the most powerful.
Not unlike the sorcerer, the assassin has a shield using purple mana. What makes it even better than Ice Shield for the sorcerer is that if you have under 13 mana when you use it, it doesn't end your turn. On top of that...
Attack: 15/10 (yes, it's that good)
The spell which gives you almost impenetrable defense also doubles the power of your main attacks. If that wasn't enough, you can use Disarm which doubles it yet again. At level 50, you could easily be doing minimum 50 damage on each attack. The final spell, which destroys all purple effectively does 8 damage per purple. On big maps, you can easily get over 200 (I had 240 recently) in damage.
In addition, the assassin only weapons can poison. The best poison damage is quite powerful and cannot be blocked. On top of that, the poison can stack, so you can use two poison weapons for double the damage. Not that you really ever NEED to use your weapon, but it's nice when you get enemies that jack their defense up really high.
Gaining Mana: 6/10
Have to somewhat rely on items for this and just luck at the beginning. The only reason it isn't lower is because the strikes morph the mana into purple, allowing you to easily get purple mana. You can generally get enough fours in purple to give you time to build up the others.
The only real weakness is that starting up can be a bit slow. Although she cannot stun, her ability to chain together attacks (highly damaging ones at that), make that point almost moot. On big boards she absolutely destroys. With just a tiny bit of luck, no enemy should get more than one turn on a large board.