Flying artillery is a pretty hard type of monster to deal with, particularly at low levels. I'm not suprised a level 4 party would find it near impossible in an open field. As the party levels though, it becomes easier to deal with.
This is true. The other thing is simply encounter design in general.
PCs in an open field (with few ranged options) vs. a blue dragon strafer?
Very hard.
Same PCs outside a tall castle against kobolds on the walls?
Very hard.
Group with no Arcana-trained character against a magical trap?
Very hard.
If you give
your group any encounter that
your group doesn't have the "tools" to meet, it's going to be very hard. I don't know why this type of discussion keeps popping up everywhere I go, in every game system I play. There's no need to come up with a specific answer for the particular challenge, because there will always be another encounter which can be designed this way.
If I was playing this scenario, as a player I'd assume one of these things (in no particular order):
The DM must not really intend for us to fight this monster. We are supposed to run away, or negotiate, or something. We don't have the tools for this.
The DM is running a deliberately deadly encounter to encourage us to be creative in surviving it. He has hidden (or will allow) "plot tools" to help us overcome this threat.
The DM is a doofus and hasn't thought this encounter out. We are all going to die because he is dumb.
The DM wants to kill us, but "seem fair" about it, rather than just having a Tarrasque eat us.
I mean, if you put your group in a situation where you
know they don't have the tools (and aren't going to be allowed to find/create the tools) to succeed, you either want them to fail (which is fine for story reasons), or you are a doofus, to me.