LANING CORE Warden is an excellent harasser in lane, since his Flask can reduce stamina and his 3 can punish enemies using all of their stamina to reposition. It forces the enemy to play defensively, which is exactly where you want them.
LATE LANING Maxing 1 first ensures that your harass has high uptime. Combo'd with Quicksilver Reload makes for a very annoying, very consistent amount of damage you can toss while also letting you continue to poke at them with your gun. Healing Booster and Healing Nova further reinforces you and your teammate's survivability making it difficult for them to flip the situation on you.
SHIFT TO ROAMING/TEAMFIGHTING By this point in the game you're a very potent supporter with a loadout that both keeps your teammates alive and makes you one very annoying rodent to deal with. Grab boxes and vending machines while traversing the map and look to gank other lanes.