I have finished the training and sector 1 levels. Now I am in sector 2 and playing the Base. It is getting harder. It took me several hours to pass Level 12. Now I am on Level 13 and it is boss fight time.
* Spoilers ahead *
The boss is a 4 level tall, Colossus. The best solution I found so far is run like hell, climbing up to the top, then use shotgun (mine is level 4) to beat him up. I was very close to kill him this morning, but failed