Omikron was hard.
I don’t know how many attempts at it I had, but only two of them got me as far as the boss, and most of them failed on the first level. I later learned not to stay in one place for too long – the baddies crowd you. Once I realised that the best tactic was sweep round the level, and when it gets busy move away for a bit, it was much easier.
Sigma, which followed Omikron, was a walkover in comparison. I limped into the boss fight on my first attempt, with no spare lives and just a single hit point. Thankfully, I’d just done one of the great bonus levels and used the credits won there to beef up my craft, and that was enough to beat him and win the game.