i think the difficulty comes mostly from the plane we were given to use,if there was a faster more manoeuvrable plane then i would expect to blitz through the level.in theory ;)
one recommendation,if you are going down the unlock planes as you play through route (whether by money/points/xp) i humbly request once the game is complete there will be a select mission with any unlocked plane option.of course the problem with that is there will probably need to be a lot of planes to unlock depending on game length,which will be a lot of work for you.