Try to get your cam at maxrange, trafe with E and aim quickly with your mouse, it's a pretty easy complex, yet kind of hard to execute at perfection, you'll get it eventually, it's just as playing an FPS (kind of).
Once you get the grenadeer trait you'll start to deal a truckload of damage on area, try getting lots of power/precision and focus a bit on condition damage, there are a lot of traits in the power tree that will give you some conditions upon critical hits (oh yeah, 20 stacks of vulnerability without much trouble )
Now to the trick part where the tageting begins, if you want to throw the bombs just to where you're standing while moving (let's say you are moving left) you have to put your cursor on the left part of your character so the bombs will be thrown at the point you casted it, when the cast is done, the grenades will fall on your legs.
If you want to throw them at your left while you're strafing left, let the cursor right below you and start using grenades, the cast will allow you to "drop the bombs" right at your side (left side).