I know this mission, It's a real scunner!
It took me a few rebuy screens before I managed to complete it.
First of all, how not to do it. Don't run away from the ships hoping that the cops will show up and give you a hand to kill them. That does the trick of killing off the ambushers, but you'll find that you'll have lost the canisters as they'll have disappeared off your scanner.
The second way is the most sensible, if slowest and boring.. Don't bother with limpets. Go in. Scoop up one cannister and get out of there while you're still alive. Wait for the mission to respawn and repeat.
The third way is the one to use if the ambushers have already killed you several times and you really want to get your own back. If you are going to kill them make sure you've got the best shields and boosters. Go rush them whilst popping shield cells like they're candy. Go for the weakest ships first. It cuts down on the number of weapons hitting your shields, and hopefully by the time you're facing the big ships the cops will have shown up to give you a hand. Make sure "report crimes" is on.
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Oh and another way... ask for help. Get someone to wing up with you.
The fourth way is to just drop the mission and get another.
I'm quite familiar with these too after being jumped a few times. The OP may no longer care, but for those still hanging around the thread looking for advice this may help. On salvage missions I'm sure theres a skill level threshold but I don't know what it is, but I know there's a reward threshold and its somewhere between 400k and 500k. If the mission reward is below 400k, I've never been ambushed. Ever. Over 400k and its a maybe, over 500k and you're in for a merciless beating, every time. So contrary to all mission board logic when it comes to these missions I only take the lower amounts. Fast, easy no hassle.
Like most things, I also have a ship tailored specifically to this task. She's my courier/salvage Adder, enhanced and rank one dirty drives, and she boosts about 560. If I'm scooping salvage and blips appear on radar, I don't wait to see what they are, cargo scoop retracted, boost, and I'm gone.
Also I don't know if someones brought this up the OP says the ships drop in on you. They don't. They're actually waiting in a stealth/silent mode which they can only stay in for so long. If you go for the cargo, they will be all around you. Sometimes they are triggered by you deploying your cargo scoop. If it is at all doubtful, do not go for the salvage. Wait. In fact, back off a bit. If after a minute or so, deploy your cargo scoop and if nothing appears you are probably safe, but still approach cautiously.
Chris Simon says alot of these people go wrong putting too much faith in their A-rated Pythons. This is true. Truth be told, it is not that great of a ship. It does alot of things OK but nothing truly exceptional. The only thing I'll add on top of his statement is not Python related, but this:
Alot of people ragequit simply because they've tried something new and did't stop to think "What do I do if I get attacked?". The list is endless. New mission types, trading, exploration, mining, dropping into a USS, fuel scooping, opening the galaxy map, getting a cup of coffee, blinking. $@#%, just ferrying a ship from one system to another can get you killed. You must
always have a contingency, equip properly, plan for, and fully expect to get your derriere handed to you every time you leave the station. BE READY.