With this build you will have alot of mana regeneration right from the start.

This allows you to play extremely aggressive with your Q. Focus on harassing until you start getting big damage.

Once you get your Tear it is nearly impossible to go OOM unless you abuse your Q when it's not needed. Always place shrooms as soon as you can and try to place them on spots where enemies often pass (duh).

If done right, in mid-late game there should be mushrooms everywhere, which deal tons of damage.
Remember that this build gives enough CDR to make the ultimate cooldown only 16.2 seconds, so you'll be placing a ton of these shrooms.