The phrase "hard at it" means to be working diligently and with great effort. However, there are several other synonyms that can be used to convey the same meaning. For example, one could use the phrase "knuckling down," which means to focus one's efforts on a task. Another alternative is "digging in," which conveys the idea of putting in a lot of time and effort into a project. Additionally, "going all out" is a great expression to use when someone is giving their all to a task. Finally, "putting one's nose to the grindstone" suggests that someone is working hard and is focused on achieving the best results possible.