Take another look at the sentence:
At the video arcade, Jeremy wished that he had slew the giant mutant tomatoes on level one. Then he would not have fought the kamikaze pickles on level two and could have kept his grape bombs for the ninja carrots on level three.
The first verb, had slew, is the problem. This verb requires the past participle. Had slain is the correct verb.
|Verb||Simple Present||Simple Past||Past Participle|
Could have kept, the verb you chose, is perfectly correct. Before you continue, review the rules for irregular verbs.