This achievement is most likely hard to get in a regular level and is likely easier to get in Survival Mode. However, it would be quicker in a regular level if you did have the skill to do it that way. 300,000 score must be obtained in one level in one sitting (no exiting and resuming). The achievement unlocks the instant 300,000 score is surpassed.
Just go slow in Survival Mode, focusing on collecting corn as well as surviving. Obviously collecting power-downs would increase you multiplier and potentially make obtaining this achievement quicker, but they make it more difficult and you may die more frequently. A good balance is best.
Some detailed tips/advice from Avenged Remains can be found HERE.