Here are some of my tips for defeating the Storm King:
BE MOBILE: You have to use purple areas on the ground to move around. This will help you escape zombies and get good angles on the Storm King's weak points.
USE MOUNTED TURRETS: Mounted Turrets are amazing and they are capable of dealing massive damage. I suggest you to use them when attacking the horn. Since they don't drop very often, make sure you use them wisely and fortify the structure it is placed on.
SMG IS DEADLY: While Assault Rifles are good, they can't compare to SMGs when it comes to DPS. Use two or more Submachine Guns to quickly weaken the Storm King while his weak points are exposed.
DON'T PICK UP SHIELDS: Inventory space is limited and I don't suggest you to carry shields in it. You get shield points for eliminating zombies, so you should save your space for other healing items, such as Chug Splashes.
CHUG SPLASHES ARE AMAZING: Not only do they restore health and shield, but they can also be used from range and they can keep your knocked down teammates alive for longer periods.
STAY ALIVE AND HELP YOUR TEAMMATES: This mode is about attack, but your attacks won't be effective if you only have two or three players on the team. Avoid attacks by the Storm King, escape zombies and take them down from distance, and revive any fallen teammate.