Simple enough concept that has been done a number of times before, but it is done well. As mentioned, it is very hard to actually select an upgrade as opposed to randomly clicking one while you're attacking. Other than that, the gameplay is smooth, so kudos!
Only suggestion I'd have for ye is that I only really saw 3 kinds of upgrades; guardians, random bolt and bomb? Could be interesting to consider the different elements behaving very differently; like rock and/or water be used to manipulate the map, whereas fire and wind are more directly offensive, like how the upgrades are at the moment.