Plants vs. Zombies Garden Warfare 2

The blue gnome king is the easiest. Purely because of there being less minions spawning.
I developed a few little tricks while fighting these guys that helped me survive with plenty of health remaining. Here they are for the blue king!
Edit: The king is located in Infinity Time mode which unlocks after you finish either the plant campaign or the zombie campaign. The first time you enter is free; from then on you have to pay stars to enter.
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Firstly, FOCUS THE KING! After you do enough damage to him he calls in reinforcements. Save your and powers to deal with the little guys if they start getting overwhelming. Pound that power into the kings face! Once he dies, the minions disappear and the game ends. At this point, if you want to get more infinity shards do the opposite! Focus the minions and avoid the king like the plague. There is a finite amount of minions that spawn and they're easy enough to take care of at this point without worry.

As for fighting tactics for the king himself:
- When you get too close to the king, he shoots out a constant beam of energy not unlike that shot from your dino (if you're doing this as a plant) that eats your health away pretty good. Try to keep your distance from him at all times.
- When you're far away he shoots slow high damage balls of energy at you. SERPENTINE! Stay outta the way of those suckas! Worst case scenario you can just hide behind a rock until the king cools down and stops pumping at you so you can get back in the fray.
- USE THAT POWER! HEADSHOTS DO MORE DAMAGE! I found if you aim slightly above his head you get more criticals.