This is a guide to Arthur's Challenge. This is recommended for levels around level 30+. This is a really good strategy if you follow my way of doing it.
Pick all of the plant cards first and then use sun cards, power-ups, or plant food. If you lost a lawn mower, pick it instead.
Magnet-Shroom (if there are kings, bucketheads, or knights, otherwise Winter Melon)
Infi-Nut or Tallnut
Power Lily, Imitater Cherry Bomb, or Chili Bean
Knight Zombies If there are early knights, use a Magnet-Shroom to weaken his strengthness.
If you don't have enough sun for a Fume-Shroom place an Infi-Nut to stop him. Soon you will have enough sun for a Fume-Shroom.
Four flag levels with them are a real pain. Use Cherry Bombs to kill groups of Wizards. If your lawn is filled with sheep, unfortunately you need to use a power-up to defeat these guys.
If there are a lot of kings, especially with four flags with basic, cones, and buckets, you need two columns of Magnet-shrooms. Also use Cherry Bombs to kill them.
Dark Ages Gargantuars
If you are playing along with other difficult zombies it would be hard to battle them. Use Cherry Bombs and plant food to help destroy those powerful zombies. Four wave levels are really gonna suck.
Start off with planting a Sun-Shroom and use plant food on it.If there is only one wave, plant a column of them. If two or three flags, plant two columns. If there is four flags, plant three columns. Plant as many Puff-Shrooms and Grave Busters ASAP when they are fully recharged. Use them to kill early zombies.Use Cherry Bomb to take out groups of zombies. Once you have enough sun, plant 2 columns of them(if there is 3 or 4 flags, plant three columns of them. When a Buckethead, Knight, or King comes, quickly plant a Magnet-Shroom. If there is 4 flags, plant two columns of them. Then plant a column of walls. Use up more plant food if all the slots are full and when you are in trouble.
If you need help please ask at the comments below.