Damage Deckard was not ever meant to be a thing. Don't be surprised if your damage numbers are low. HOWEVER. This build I have had great fun with, getting a lot more kills than other builds especially after lvl 16.

Throw your potions off cool-down around your team when there is no team fighting. This will reduce likelihood your team mates getting killed from a gank. During fights, try to throw potions at team mate's feet to keep them going.

You have power in controlling how your team plays. If you throw potions behind your team, they will likely retreat. If you throw in front, they are more willing to dive. Keep that in mind when you team fight.

Throw out an E ahead of where enemies are running and throw a W on top. This will increase your chances of landing your E and stacking it as soon as possible. This is your bread and butter combo.

Stay near your team mates. You get 10 armor when allies are in your circle and that's enough to make you a semi tank. Practice taking some skill shots for your back line. It makes life easier for them and you can drop an Alt+Q to heal yourself up (much more reliable than throwing a potion at them and hoping that they will pick it up).

Level 1: You're getting scroll of identity for that 15% armor reduction. Complete as soon as you can.
Level 4: Potion of shielding is just generally better during team fights for saving people when they need it
Level 7: Cube mastery for more scrolls and lower mana? Hell yea!
Level 10: Lorenado is my preferred. Allows you to move during your ult and is basically a ranged falstad gust in slow motion. Good for cutting off choke points for escaping and trapping the opponents. The 1 second delay is something you need to get used to predicting. Plus you can knock em closer to you so you can root them for your team to kill.
Level 13: Ancient blessings is good for teams which utilise AA or you have a bunch of melee. If you find you are lacking in healing and people aren't picking up your potions, this is a great way to keep them alive in a short burst.
Level 16: Stone Curse BIG power spike. They won't know what hit em when you do that 200% bonus damage, root them, and lower their armor.
Level 20: MORENADOS! This is so that you can continuously push their team around and into your scroll of sealings