When using Poison Cloud on a single opponent, get close in and target the Cloud at whichever of the opponent's feet is nearest to you. If you do that, most times the Cloud will hang around your opponent and do repeated damage. I had a fully developed lich doing this, and the megadragon was suffering up to 800 hit points of damage from a single casting.

On the other hand, you might catch a series of opponents lined up one behind another - like in the Training Hall if you catch the Lizard Orc warriors at exercise. You may then want to cast Poison Cloud straight ahead, so that it goes down the line, hitting every monster it reaches.