1. Be Patient

Tower defense games, at times, can be very frustratingly hard, annoyingly slow and highly reptitive. Super Sanctum TD is no exception. In dealing with all these negative traits (at least, for some players) of a tower defense, being patient is a must.

You may have failed to protect the core on the last wave, be patient.
You may want to speed up the game but keep missing the resource crate, be patient.

2. Experiment

If a defense build and maze pattern that have worked on previous levels doesn't work on the current level you're stuck in, then you have to experiment. Try other maze patterns, skill arsenal, tower choices and perk builds.

3. Guides?

When the going gets really tough and the level or the game itself is taking too much time than you intend it to be, seek for help - that is, if your gamer's pride allows you.

Personally, I sought out help and found Sunset Haste's Hard Mode Gameplays on YouTube. There, I found out the Lightning + Flamer tower combo. And since then, I've been finishing Hard Mode levels on my own.

4. Enjoy

Savor them waves being killed. :D