expand_less Mikasa performs significantly better when he has sufficient time to prepare for encounters.
For example, if he expects a battle, he can draw a magic diagram at a safe location.  He can do this as a full-round action by taking 10 on his spellcraft check, or he can use a full minute by taking 20 (for difficult battles).  Once the diagram is drawn, he can lie down inside and spend a minute to separate his soul from his body.  This allows his soul to fly about as though it were a ghost, while his body lies comatose within the diagram.  He can remain in ghost form for up to four hours, but he can only activate this ability once per day, so he must choose his battles wisely.
In ghost form, Mikasa cannot interact with the world in the typical way.  His incorporeal hand simply passes through physical objects.  Likewise, he can fly through obstacles or up to 5 feet into solid barriers (such as the ground or thick stone walls).  Whenever possible, he uses this to position himself for maximum advantage.  Areas of darkness or dim light work best with his dark sphere abilities, as in these areas he benefits from concealment and his meld abilities (such as Dark Slaughter).  When he finally does emerge to attack, he'll need to make a stealth check to catch his enemies unaware.
To meaningfully interact with the world in ghost form, Mikasa must rely on his telekinesis.  By tapping into his key emotion and gesturing with his arms, he can lift up to 500 lbs. of loose material with this magic.  This can be as a single object of up to large size, or several smaller objects (max 9 total).  Typically, he would bludgeon his enemies with whatever materials are available, with either a focused snipe or a barrage of attacks.  When necessary, he can recall his Forbidden Lore (w/ Mind over Matter) or spend a spell point to increase the potency of his telekinesis, but he should beware of psychic backlash when pushing himself to his limits.