Perhaps I should have counted down waves instead of up. Wave 6 is the last (does say on the game description page :P, but literally no one reads those, I'm no exception there :P).
Do need to employ some classic Space invaders tactics to win, such as firing up, through the deffences,.. or simply leading the attack and firing in front of them. Had contemplated giving even more lives, but through game testing I felt 20 left enough of a challenge to feel a sense of achievement through winning.
Yeah hold for fire was a must I felt, especially when getting the fire speed to max :P.
thanks for playing and commenting, much appreciated.