The essay-grading software I've seen described in enough detail to figure out how it works has a heavy bias towards enforcing specific structural rules, while mostly missing "quality of argument" type questions. So a student writing to exploit its bias would want to worry more about formal structure: essay organization, correct grammar, use of certain "must-have" keywords, etc.; but worry less about things like avoiding logical fallacies in their argument, because the software won't notice those.