Let's just assume that the attacker has 3 attack dice (with no actions or bonuses)
That is an expected number of hits of 1.5 hits.
Three agility means the TIE Defender will roll 1.125 evades on average.
This is a little crude (and not entirely accurate, but its close) to about .375 hits per shot.
It takes 6 hits to kill the TIE Defender 6/.375 is 16 shots. It is going to take about 16 shots from your x-wing or b-wing to kill the TIE Defender.
Let'd compare that to the YT-1300
1 agility means it will only evade about .375 hits per turn. Meaning the YT-1300 will take about 1.1125 hits per time it gets shot at. (let's assume it is the 13 hit version). 13/1.125 is 11.5556 shots.
It takes about 11 1/2 shots to kill a (Character) YT-1300 and 16 to kill a TIE Defender.
When you consider that a TIE Defender is significantly cheaper than (a 13 hit) YT-1300 a TIE Defender is considerably better at staying alive.
B-wings and Y-wings only have 8 hits. 8/1.125 is 7.111 shots to kill them.
A Firespray with its 2 agility and 10 hits takes about .75 points of damage per shot it takes. So it dies after being shot about 13.3333 times.
I think it is pretty much accepted that a Firespray is about the hardest thing in the game to kill. Well a TIE Defender will be harder.
Note: I am aware that my calculations are over simplifications and that expected damage can not actually be calculated by simply taking expected hits rolled and subtracting expected evades rolled (since you can never take do negative damage). But these calculations are not intended as anything but a comparison between how ships on how easy it is to kill them.
Edited by Hrathen