His magical damage is is painful on the early-mid game, so you must be aggresive on the early-mid game to reveal his maximum potential.
Does a lot of magical damage No allies can right click the enemy while you ghost form him.
An exploding pulse deals damage to enemies and structures in the area. Deals 33% damage to structures.
Nether Blast is your skill for farming and killing for early game. The AoE damage is considered high and can easily destroy a creep wave. It is also a very good harassing skill, with 325 damage in level 4, your early game will be easy. Can also damaging building, so it is a useful skill for pushing towers.
Make a target become a ghost form. It can attack nor attacked, but it will receive more damage from magic attack. So you can amplify your nuke damage. So cast this on a target and then use Nether Blast and also Life Drain to increase the damage. Can also save yourself from physical attacker by casting this to you or to the attacker. Can also saves you from Juggernaut's ultimate because it will not target a ghost unit.
Deals damages to someone that cast a spell. The damage is high and can make the casters feel the pain. The AoE is wide, so you can hide this ward from enemy's sight and make them cast spells and feel the damages.
CHANNELED - While channelling Life Drain, Pugna siphons HP of the target enemy unit. Lasts 10 seconds. Upgradable by Aghanim's Scepter.
This is your most powerful nuke, you can cast this anytime with Aghanim Scepter so you can always drain tons of HP from your enemy. Use this in a teamfight and start draining enemy's HP until he death.
Why maxed Nether Blast first? This is your main nuke, you will need this for ganking, farming, and killing on early mid game.
Why maxed Decrepify before Nether Ward? Decrepify.is more useful because it will amplify your magic's damage so it will be easier to kill your target. And level 1 ward is enough for early game.
Early game Pugna is one of the easiest early game in Dota. You have an aoe spell to destroy a creep wave and also damaging your enemy on your lane, it will be so useful in laning phase to get multiple last hits and also harassing your enemy. And if someone is trying to gank you, you can use your Decrepify to somehow flee fro the gank. Maybe you can cast the Decrepify to your chaser and make his movement become slow so you can run away.
On mid game, usually people start to gank each other. What you need to do is to find the ewasy enemy to start killing for your team. Go to the target's lane and use Decrepify + Nether Blast + Life Drain to kill the target. And always tell your friend to help you because maybe your combo isn't enough to kill your target so your friend can help dealing more damage.
On the late game you are become stronger than before. I assume you already have the Aghanim Scepter and ready to demolish enemies in the teamfight. Before the teamfight start, place your Nether Ward in a secret locations and make sure your enemy can't see it so the ward will deal damages and your opponent can't see it coming.
You can use Decrepify to their physical attacker and make him cannot attack your team, and then you can use the Nether Blast and also Life Drain to damaging your opponent.
Why Aghanim's Scepter? Decrease the cooldown on your ultimate. Makes you become a decent initiator for your enemy team.