Tips to Use Enigma (Dota2)

• When starting a hero battle, if possible try to cast Midnight Pulse first, then Black Hole. The four seconds used while you are channeling your ultimate, you will have also drained their HP by 12%/16%/20%/24% respective to the ability's level. You should animation cancel the Midnight Pulse to give your opponents as little time to react as possible.

• Keeping Eidolons around you is a very good idea when there is a Pudge on the enemy team, as it makes you very difficult to hook, and Enigma is very easy prey for Pudge in the early game.

• In sufficient number, Eidolons can be used effectively in encounters, as en masse their collective attack does significant damage. If you die in an encounter and you have Eidolons still alive, use them to boost your team's damage a little.

• Black Hole's cooldown is very long, so you must judiciously use it even in prime situations. Although it should ideally be used on four or five enemy heroes, this is an uncommon opportunity, and using it on a couple of enemy heroes or even one important one is usually justifiable. Using it wisely does not necessarily mean using it on as many targets as possible, but adding to the team fight in the most significant way possible.

• Enigma's presence in encounters is greatly diminished while Black Hole is on cooldown, but he can make himself useful by saving Malefice for a crucial moment and also sneaking around to use Midnight Pulse within an area the enemies may try to escape through. A good combo is to use Malefice on an enemy walking through Midnight Pulse.

• Enigma works best with allied heroes who have strong AoE capabilities, such as Warlock or Queen of Pain.

• Enigma can kill Roshan before the 10-minute mark, with the help of Level 3 or 4 Eidolons. For acquiring them, use Demonic Conversion on either a neutral creep if he's on the Radiant side, or a lane creep if he's on the Dire side. An item commonly picked up is a Medallion of Courage, as it allows him to reduce Roshan's armor for a higher damage output. Try to tank the first few hits in order for the Eidolons to multiply, then use Demonic Conversion on one of them. The damage output, without Enigma's help, is 319 (with Medallion of Courage, 440), allowing him to take down Roshan in less than a minute. 

source: http://dota2.gamepedia.com/Enigma