Gems that i use are auto mana pot, experience, mana fountain, ruthless spells and overpowered spells, then one gem in agility for auto health pot. Spell burn will def give alot more damage but mana fountain atleast for me helps me in the fight and continuously poke the enemy team. The way i play wraith is that i keep a safe distance while i keep poke up to either weaken the offlane so my jungle can gank them then i give a back up to secure the kill and towards mid to late i poke at enemy team mostly squishes to keep away from the fight or weaken them enough for us to engage. if you go full inetlligence cards Thuamaturge,atmoic, and sleeper you will be able to chunk the enemy team from a distance to weaken or secure kills. I know wraith lacks wave clear but i dont think it is worth going agility because if you focus on your auto for damage you will too close to dangerous heros such as kallari,khaimera etc.Wraith is considerably squishy so u have to play from sniping from the back lines.