Yeah, Flash is really hard to hit. Your choices are either to keep dodging and using decoys till he runs out of missiles and AB fuel and then take him down, or hit him early by shoving a pair of IR missiles up his tail.
As for Thrahkath, I usually decloak right behind him and pump a pair of missiles up his tail, so I haven't seen much of how he flies in WC3. My vote for who is the hardest to beat is Hobbes--you have to use your greater maneuverability against his heavier guns.
Iget it! I went back and started the game again. I always thought the empty seat was mine. You know, im standing up giving the brief, that was the chair I would sit in if someone else was talking. Flash isnt a very good wingman though. He doesnt listen. which doesnt fit with what he said after I beat him (7 tries)