I tend to ignore (well, not totally ignore) the Sarthas and just zigzag away from the Ralatha until it is out of range. IIRC, your rear turrets will still fire, although not as fast or accurately as if you were doing it yourself, so they might kill one or two of the Sartha. Once I'm out of range of the Ralatha (i.e. its flak cannon aren't chewing away at my armor and I'm not going to get hit by a stray amg shot) I'll turn and kill the Sartha. I personally prefer to use the front guns - those three mass drivers do a LOT of damage. Using the turrets means that you have to fly straight, so you'll lose a lot of shields and armor if you're up against multiple enemy.
Once the Sartha are dead, just do what Wedge009 suggested - wait for your shields to recharge, lock torps outside range and head in. To dodge the AMG, don't head straight towards the Ralatha. There is a range over which the torp will still maintain lock. Head in at one extreme, then once the AMG fires immediately change to the other extreme. You should be able to dodge easily if the AMG fires at long range. ALWAYS dodge the AMG, even if you have to lose lock to do it. It's better to abort and just have to wait for your torp to lock again, then to take the hit and have to wait the X minutes for your shields to recharge. IIRC the Kats will every so often fire a very wayward shot, so use that for an easy run.
To dodge the AMG on the way out, either just change course every few seconds and trust to luck, or change to your rear view, then switch forward and change course as soon as the AMG fires. You can even plug away at the Ralatha with your turret guns - doesn't do any good, but it might make you feel better
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Just remember - always keep your shields high and dodge the AMG at all costs. Piece of cake.