Someone can tell me how I can defeat him please?

What class are you using?

I've always found that hitting him until he falls over works well.

More seriously, it's not that hard a fight. As usual, don't stand in stupid (i.e. move out of "telegraphs" that show where AoE effets will land or have landed), interrupt "interesting" and heavy enemy channels / casts if you can, and avoid attacking if he starts reflecting damage. (I don't remember him reflecting, not like Vaylin does.)

One thing that will make it harder is if Malgus is glitched and won't heal you. I don't know of a good fix for this, but it's not impossible even then. I had that the last time I played through it - my character was a Vengeance Jugg in full 230 with no augments, and Malgus just plain wouldn't heal me at all. It raised the diffiulty of the fights along the way more than it raised the difficulty of the Gnost-Dural fight.