The time you should have Mejai's Soulstealer is at level 6 so you can just Ulti down enemy to gain stacks. If the game isnt going your way no point getting this item

Keep Casting Lay Waste to get your Tear of the Goddess to give max mana!
When you go back and your in the base just cast all your spells so youll get your mana back.

Wall of Pain decrease's the Armor,Magic Resist and Movement speed of enemies and it aswell works like an ward so you can just cast it to bushes so you wont need to facecheck great spell

Defile makes an AoE under you and you can move while its on. If your HP is fine you should go near enemies and keep it on and casting Lay waste

Cast Requiem when enemy is escaping to get those kills. But be sure it will give you kills or atleast assists cos it has long cooldown