Take a look at SPG-12 for the T55a.
You need to stun for 140 seconds AND enemies need to kill 2 tanks stunned by yourself.
Assuming each stun averages 13 seconds, you need to hit with 10.7 shells. So basically, 11 shots.
When a shot and it's reload takes 45 seconds, this means you need 495 seconds of continued fire. This doesnt take into account any sort of aiming time whatsoever, which in reality could add another 6 or 7 seconds onto each shot. Dont forget times where there is a lull in battle and nobody is spotted, or the only ones spotted are behind cover. You could sit there for another minute with no target.
This means, without even considering aiming time or a break in the fighting, you need to hit 11 tanks and it'll take you 8 minutes 15 seconds to do this. If you dump aiming time onto that (plus the time when no target is avaliable), you're talking 9 minutes minimum. Probably pushing 10+ minutes, in battles which are what, 13 minutes long? Dont forget another minute where you need to get into position at the start of the match.
How many battles actually last 9 minutes? I'd say hardly any. You also need 100 accuracy on all 11 shots across these 9 minutes.
Oh and don't forget, if you do actually manage this, you're relying on your team to deliver a killing blow in that 13 second stun window… TWICE. Actually killing the enemy yourself can hamper your success.
Now I appreciate, there might be times where 2 or 3 tanks are clustered together which will reduce the time, but what i've posted here is a base minimum requirement. And it's fucking absurd.
Seriously, it just screams at the fact that WG makes missions as hard as possible because they hate giving away free stuff. I don't see this in warships, the free ships can be a huge grind, but it's possible. These missions are so absurdly complex and difficult, that you stand a better chance of winning a lottery. Don't forget, this is just one mission i've picked at random. The LT missions and even some of the HT ones are even more insane.