Not sure if this has been asked before but as I've started replaying the game for a third time, thought I'd ask! In order of frustration then:
1. The one where you have to chase a Learjet with a Cessna. This is virtually impossible to do even if you get above the plane and swoop down towards the corona. Only ever managed to do this by altering the aircraft config to make the Cessna aka Dodo a bit quicker.
2. Any of the timed missions, but the one in San Fierro where you have to chase the four motorcycle couriers and retrieve their packages. Even if you bend the rules and snipe one of them beforehand, it's still a struggle to make the time limit.
3. Zero's New Model Army. I can only get this one to work if I manage to bomb Berkeley's helicopter at the start, otherwise there's just too many barrels to contend with among all the other tasks.
1. The one where you have to chase a Learjet with a Cessna. This is virtually impossible to do even if you get above the plane and swoop down towards the corona. Only ever managed to do this by altering the aircraft config to make the Cessna aka Dodo a bit quicker.
2. Any of the timed missions, but the one in San Fierro where you have to chase the four motorcycle couriers and retrieve their packages. Even if you bend the rules and snipe one of them beforehand, it's still a struggle to make the time limit.
3. Zero's New Model Army. I can only get this one to work if I manage to bomb Berkeley's helicopter at the start, otherwise there's just too many barrels to contend with among all the other tasks.
I didn't realise that you could bomb Berkley's helicopter, it's gonna be too easy now the next time i play through it 
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