*Sees Cosmog's stats and movepool* I'm going to guess this is why people are saying you'll want the EXP Share on. However the end results are looking worth it...
High HP and Attack, good Special Attack, nice bulk, its just the Speed that's a shame for not reaching that 100 benchmark. Though compared to other Legendaries you can do worse.
STABs with Sunsteel Strike (or Iron Head if you don't want the low PP) & Zen Headbutt; other Physical moves are Crunch, Flare Blitz, Earthquake, Stone Edge/Rock Slide, and Wild Charge; if you go for mix its Specials are Flash Cannon, Psyshock/Psychic, Thunderbolt/Thunder, Flamethrower/Fire Blast, and Snarl. Finally utilities are Cosmic Power, Morning Sun, Noble Roar, Wide Guard, Work Up, dual screens, and Thunder Wave.
Solgaleo has what it needs for going all out, well aside for something to increase its speed (aside from Flame Charge, though I was thinking something like Autotomize).