In good hands Veigar is very powerful. At first level he must always finish minions off with his 1st skill. As soon as he runs out of mana use clarity to stay on the layne and get more AP by finishing minions off with 1st skill. For harassment u must wait until you get 1st to 3rd skills ready. Use stun on the enemy and dark matter then use 1st skill on the enemy and run away. In case the enemy is an OP DD like Tryndamere predict his movment and use dark matter behind you where he will step trying to get a kill of you. In case Tryndamere wont stop he will have less hp if hit by dark matter what will make him easy kill using 1st and ulti together but be careful as some champions got skills that avoid death or skills (Trynda ulti/Kayle ulti/poppy ulti/sivir spell shield). Good luck playing Veigar. (In case of dying to much u can change Void staff for Cristal scepter in case no enemy has magic resist and/or you can replace Hourglass with hextech revolver that grants you spell vamp/damage/life steal though damage/life steal does not seem like a good idea it works perfect via spell vamp as you can easily heal by using dark matter on a monster/minion and running away if it's fully upgraded it will kill them instantly so there's no need to wait for it to land) .