My 200th answer :D
Well when i faced him i had a problem with its ominous wind and i was lv 85 as lucario when i faced him so i would suggest getting to a high level seen as he is probably the hardest boss out of the special missions.
There are really easy ways of beeting him though all may be hard.
.agility is great it doubles your whole teams speed use that a few times to attack 3 times in one turn by the time hes reached you.
.quik seed works just like agility just not as well.
.My personal favorite- use a pokemon such as maskerin with access to silver wind and ominous wind if they use that a few times there stats will raise as well as do lots of damage.
.save up with lots of reviver seeds
.use basic seeds like violent seed, sleep seed, x eyed seed all the basics
.swords dance and other stat raising moves
.take maxinum pokemon
.maybe find ice types at places such as cervice cave
.if you do use the silver wind and ominous wind strat link them so it can use two moves in the same turn that can damage girintina from any distance as well as gt lots of stat boosts.
.type advantage moves
.i cant think of much more but get your pokemon to a high level
it took me a (cant remember the exact lvs) lv 85 lucario, lv 80 donphan, lv 70 uxie and a lv 75maskerin
Recruitment rate is 8.2