Its important to play carefully, dont go so far from the tower and start taking the enemys health little by little.

when the enemy got low life, like 50% then you can use (very very fast) flash, malefic visions, nether grasp, and when your ulti is working then hit him/her with ignite, if its necesary use call of the void a bit farther from the way that the enemy is running away.

Seal mana regen are very important because you can spam your abilities for free, then, malefic visions regen your mana, when a minion/champ dead you win abaout 30 mana, so if yoor skill transfer enemy by enemy your mana will be complete and you never stay out of mana.

Thats the things´ order but if u want to can take the mejais´ soulstiler, you can but first take the tear of the goddess.

if you are playin vs a twitch or katarina, get the call of the void first to stop them, ifs not try with the guide order