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 largemedium size, or several smaller objects (max 96 total).  In any case, he propels these objects with such force that they act as though one category larger when dealing damage (so as large sized objects by default).  Typically, he would bludgeon his enemies with whatever materials are available, with either a focused snipe or a barrage of attacks. attacks (up to two per round).  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.  limits.  Theoretically, he can bludgeon his enemies with objects weighing up to 16 tons, or simply lift up to 125 tons.  This would require enormous strain and concentration, preventing him from taking any other actions.
Perhaps the most efficient attack is a combination of hostile lift and bludgeon.  By spending a spell point on focused might, then four for hostile life, and finally invoking his Forbidden Lore, Mikasa can take hold of up to four medium-sized creatures and bludgeon them together in a barrage of attacks.  Each creature deals damage both to itself and its target, effectively dealing 400% more damage than his base attack (not to mention fall damage).
Defensively, Mikasa depends on his stealth and incorporeal nature to avoid most attacks.  His physical body is naturally vulnerable, and he must keep it safe at all times.  If it dies, so does he, but if his ghost form dies, he may yet live for a few moments as his soul violently returns to his body.  Almost any form of disease or bodily curse is fatal to him, and so he should avoid encountering foes that can target his Fortitude save.  Even something as simple as the flu could kill him.  Luckily, in his ghost form, he is immune to most of these threats.  If he is ever attacked direction (such as by a magical weapon), he can attempt to use the catch maneuver to telekinetically block the attack.  Similarly, he can use telekinetic manipulation to subtly redirect or misdirect key attacks as an immediate action.  Should a creature attempt to influence his mind or spirit, he can reroll any Will save in a similar way.
Mikasa's greatest weakness are ambushes, surprise attacks, or any instance where he lacks sufficient time to prepare.  Without a magical diagram, he cannot use his magic (as either he or his target must be in the diagram at the time of casting).  In his natural human form, he is incredibly fragile.  In these situations, it might be best to run or hide to fight another day.  When that is not an option, he can attempt to hastily draw a diagram (by making a rushed spellcraft check), or he can rely on conventional weapons (he's still decent at throwing rocks).
Outside of combat, Mikasa is a great scout and infiltrator.  He possesses the skills and know-how to disarm mundane and magical traps, or pick locks using his telekinesis alone.  His eyesight is relatively poor, however, and since he wouldn't typically trigger most mundane traps, he should be wary that dangers could still await any allies that follow him.  His psionic mind link is usefully for reporting quick scouting missions.  Though he is soft spoken, he has a pleasant personality and charm, which allows him to socialize decently well with anyone that can overcome his grisly appearance and dual-nature.