Just to give more details on "King Galaxian" level of Pac Man World.
Your fly your ship in a 3D perspective, like Silpheed. You are flying in the space. The enemies are 3D models based on the Galaxian enemies. It starts very easy, but it gets a little harder late (Not all that hard).
Then you get to fly over some terrain, which have laser walls coming up to hit you, so you have to dodge them. At this point , lots of enemies are shooting at you. It never gets too hard, but it's cool enough anyway
Then the boss ("King Galaxian" is actually a "boss" level") is that green enemy from Galaga (The one that captures you), in a very cool 3D model. He even has the "attract beam" weapon.. he attracts you, swallowm then spit .
The boss has 4 "hit spots", and you have to destroy each one to beat it. At every hit spot destroyed, his attacks gets harder. The last "attack" is actually a bit difficult, you have lots of big bullets coming from every direction. I lost 5 lives to get past it

(Everytime you die, you get back to the beggining of the boss)