I am aware that the hard difficulty is currently too hard on some levels. While I have personally beat the levels on all difficulties, I've since made changes to enemy health and tower damage that have thrown things off balance. The next update will correct this. The hard mode was not intended to be impossible. Starting energy will most likely be increased for some levels in this mode, along with a possible across the board damage increase.
In creating Defensoid, my goal was not to make an overly difficult tower defense experience, but rather an accessible and fun one. "Beating the game" on normal should be well within reach (after a few tries of course). The hard difficulty takes much more care. The early game is critical for later success. One of the modifications I made was to cap energy gained from killing Oids to 25 (except for the Bossoid). As one user pointed out in the comments, this will only take you so far when the Oids have twice the health.
So stay tuned. Suggestions are always welcome.