Malthael is the final boss of Act 5 and as such cannot be missed.
I defeated him with a Monk, whose Seven-Strike ability does wonders against bosses when they are alone. I strongly recommend having anything with sockets containing Amethyst as this will give you extra life on each hit. Aside from that, it was spamming the attack button and using Breath of Heaven, the Seven-Sided Strike and several Whirling Kicks that gave me the victory.
I know there will be other strategies to defeat him, that's just the one I used.