Flying Tigers: Shadows Over China

Not hard at all, I got this achievement on my first try with 22 seconds to spare. the countdown starts at 5 minutes if I'm not mistaken. Just focus solely on the flags and don't engage any planes. Obviously, play on casual. The flags are more or less situated in a circle, so just fly from one flag to the next one closest by and then to the next one in the row and so on. I only got engaged twice, if so, just do a barrel roll and evade the hostile fire. Just keep flying at max speed all the time and you should be able to pop this without too much of a hassle.