Nova is useful for picking out single targets. Her snipe ability is good versus damaged or lower tier units and her mindcontrol can steal units nearly permanently from your opponents. Her Courage buff makes her good for team play as well. Use her ultimate with caution! The nuke affects a wide area and damages yours and your allies’ units as well. Try to warn your allies about it and use it, when your opponents can’t evade it.
How to Play
Like with any other hero, who has buffs, you should get it first. Just because your units are the most valuable thing you have early on, and everything getting them better is worth it. Especially +40% damage! Countering Goliaths (with Mindcontrol) is not as much needed early on, as killing enemy marines. Also your Snipe won’t be that much of a threat for enemy heroes to make it more important than Courage. And it won’t even one-shot Marines, if they aren’t already damaged.