Each square contains a different challenge, which include tasks like killing large numbers of enemies, taking on multiple bosses at once, or eliminating enemies in the correct order. (the-games-blog.com)
Some missions ran on because they were so easy that bonus objectives, particularly the ones that involve killing a certain number of enemies with a specific unit, became difficult. (monstervine.com)
You'll have to defeat bosses or a set number of enemies within a certain time. (playstationlifestyle.net)