Unfortunately, Krants have better shields, armor and manouvreability then the Scim, so you're going to have to beat them with piloting skills

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Don't go head-to-head with a Krant in a Scim. Your mass drivers do significantly more damage then the Krant's lasers, but the lasers have a lot more range. You'll have lost your shields before they even come into range.
In a Hornet or a Rapier, I will usually try to get a few head-to-head hits in. Line your target up early, open fire at about 6000 klicks (laser range is 4800, but he's coming towards you), then turn away
slightly just before his shots arrive, so that they just miss you. The Kat will usually fly straight for a little while after you turn, so most of your shots should hit while his miss (note that this technique works exceptionally well in Priv2). If you find that you are tending to miss, try aiming slightly to the side that you are planning to dodge towards, so that the Kat turns into your shots to follow you.
In a Raptor, I will often do the opposite. Again, go head-to-head with the Krant. Again, dodge slightly to one side to avoid his shots. When you close to around 3500 klicks, turn back towards the Krant and open fire with full guns. At that range your Neutron/Mass Driver combo should make short work of him and his lasers are pretty ineffective. If you can't quite kill him, you can try a manouvre I have dubbed the 'shield swipe'. Just go to ram, but turn hard early so that your fully charged rear shields make contact with his non-existent front ones. Don't laugh. I just about perfected that manouvre.
Unfortunately, these techniques really don't work in the Scimitar, which has neither a shields, firepower nor gun range over the Krant. The best idea is to try to close to short range while sustaining minimal damage. At close range, your mass drives have a definite edge. The best technique for me is to fly semi-straight towards them, as if you meant to scrape wingtips past them. Go in hard at full burners and most of their shots should miss behind you. Just before you pass them by, turn hard and release the burners. You should 'slide' a bit, i.e. your ship is changing heading but still travelling in the original direction. You may want to turn away slightly and then turn back, like a fish-hook, so that you don't collide. If you time it right, you should be able to get on their tail quickly. Then tear 'em apart with your mass drivers.
Oh yeah, make good use of your missiles. Save your Javelins for tough ships like the Krant. They're really wasted on light fighters, while sluggish beasts like Grathas you can just get behind and pound with your guns. But against Krants, particularly if you're outnumbered, they're good. Use them at close range so they can't miss. Soften the target with your guns while you wait for the Javelin to lock, then boom. Just make sure not to shoot your own missile.
Hope that helps.