Plants vs. Zombies Garden Warfare 2

* You must first unlock infinity mode through doing single player missions for the plants/or zombies. Once they are done you can enter for your first time for free. (I preferred entering as a zombie to get the cat robot as I felt it was easier to aim and control.) Gnome Kings pop up every 5 waves for you to dispose of and when you do you get full health back Gnomus the rainbow Gnome king is on wave 25 for this achievement.

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Easiest method:

Using the Easy mode glitch really makes this achievement simple. Just start a garden/graveyard ops match (online, not solo) on easy and back out once the match starts. Now you can enter Infinity Mode and the difficulty will be set to "easy". Credit to karma for this new solution.



Original solution: Ok got this one done solo. To help you do this the best advice I can give you is this. Play the game as if your playing zombies in call of duty. Move around the map forcing the enemies to line up and follow the path you lead them around the out side of the map. NEVER RUSH- Gnomes cant steal your time. Always keep moving (backwards when possible to shoot the guys following you). Do not pick up any shards unless you need them or you just have to take a path that has a few. The big gnomes with the "umbrella weapon" can only shoot a beam here and there. Keep your distance, make them fire it and when its done pop out and lay into them. As noted by tornprince2012 if the umbrella gnomes are close to you they will swat you for a good bit of damage, but if your following the guide then they should be chasing you as you maneuver around the map.
When the giant floaties come out handle 'em first and fast use your missles to drop them quick and get back on the move making a train. When the boss comes out your optimum time to strike is immediately. Wear him down until he disappears. He will have plenty of reinforcements so take your time and line them up dodging the ULTRA-huge pea shooters until last if possible. Do the same thing with the ULTRA-huge pea shooters lure them to a good location where you can shoot them around a corner or over a block without taking any/much damage.
Once the King is out finish him off asap. Do not wait as you will get surrounded quickly. If you were able to force him to disappear earlier then he should barely have any health left use your missles and finish him off with a laser.
Bottom line do not panic none of the gnomes can capture your time areas except the bosses, and if you hit them quickly enough they wont hover on it either. Most of the time when I lost it was due to time thief balloons not to the enemies. Remember take your time line them up and kill on good angles. Don't rush and save shards when you need them. You will have it in no time. I found it easier solo as you don't have a friend eating all the shards because he doesn't know how to run a train. If you loose it will cost you a lot of stars to come back (17 stars to come back to level 21). Try not to mess up but if you do you can come back. Enjoy and good luck.
Playing with 1-2 people is ideal more can make it a ton harder featuring more enemies.

BEAT 'EM ALL! This achievement pings when you beat THE rainbow king, Gnomus.
To get here you have to beat the blue king (round 5), green (round 10), yellow (round 15) and red (round 20). This is probably more easily accomplished on a group but it can be done solo. To get to him alive and with good health remember to focus power units to keep your damage minimal. If you're struggling go on a minion cull to bring back some health. Don't collect all the infinity shards you see; they will sit there until the king is dead. Use them as health sources and leave them lying around if you're good for damage control.
YOU MADE IT ALL THE WAY HERE? HAVE SOME TIPS!
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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FOCUS THE KING! After you do enough damage our rainbow friend will summon in reinforcements AND DISAPPEAR. Mini bosses will spawn to fight you. This is where it gets a little interesting.
Your tactics should be taking out as many of the little guys as you can first. Once the mini bosses die the king will spawn back in and more enemies will come and holy moly you could get overwhelmed quick if you're not careful.
Make sure to use all your buttons on the mini bosses to take them out quick without taking too much damage. If you can, try to save the for when the king spawns again. After he comes back, FOCUS THE KING AGAIN! once he's down the round will end and you move on like a champion. If you have a lot of health ignore precautions and just pump in that power move damage. As you've learned from reaching this point, your health is maxed after you defeat a king and move on to the next place.

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.

Edit: This is done on infinty mode which is unlocked by
completing the zombies or plants storyline quests.
Edit 2: Thanks to Sakori for this information that you fight a king every 5 waves wave 5 is blue then green at 10 yellow at 15 red at 20 and then Gnomus at 25.
Edit 3: Thanks to TrooperAndy7 for the following information that when you kill enemies they drop shards which give you health and also give you score you can save these by not picking them and use them as an advantage to gain health when needed.
I can't remember exactly what wave you fight the gnome King but I can tell you it's in the early 20s. I played infinity mode solo and died at wave 6 I'm not the best at the game but to be honest it was pretty hard. I tried it with two players and it was much easier. So that is my solution to the achievement play two player you could try three or four player but two player worked for me.
Some advice I can give is when it's the floatie special wave ignore everything and destroy them because if you don't they will destabilise time and you fail also be wary of enemies spawning around while you're trying to destroy them the fastest way to destroy is using the Y button ability this goes for the boss rounds aswell the Y button ability seems to take the bosses down faster too.