Pretty much what everyone else has said. But even with a Viper, don't be afraid to attack single Fer-De-Lances, Pythons, Annies, etc. Even a half decent pilot with two small beams can take out an Annie in 60 seconds just locking on to the power plant. You don't actually have to kill any of the ships, just get their powerplant down. A 97% hull Annie will blow with some fire on the powerplant.
Like others have said, don't bring cargo, this will let you choose your battles. Even in a MK III, you should be able to take some wings, like two Eagles and a Annie, get rid of the Eagles right away, and usually by the time the Annie notices you, they'll already be dead. DO NOT TAKE ON WINGS WITH A FAS IN THEM IN. In a Python or something, sure, but a Viper will get wrecked by the FAS's magic A.I. that allows it to point at you no matter what manouver you're doing.
My general plan of attack is:
Find ship: Look for fireworks in the distance, or scroll through radar signatures with A+whatever on the D-pad that I'm looking for. "next ship, next hostile, next biggest threat, etc"
Either let ship approach me and scan me while I scan him with my KWS, or trail a ship already in combat while scanning.
Either open fire with what I have after scan is completed depeneding on ship type, or wait until the scanning ship has scanned me, and turned around to leave (because I have no cargo and he left) and fire something powerful into his rear to take his shields out first. (Like PA on Lance)
Then they die and I collect the money. If it's a wing, I usually get behind the weakest target first, unless I know I can take out the strongest one instantly. (Like PA spaming a FAS do death in seconds with an Annie)
The size of my ship never matters on whether or not I'll attack a single target. Ship size merely changes time to kill. A Sidewinder can very easily take out a patch 1.5 Elite Annie. It will just take some time. Some of my favoured builds are for smaller ships actually. Like a Courier with two med beams, and one Imp Hammer for shield stripping. A bi-weave shield (because it's a manouverable ship, and rarely gets hit) and just some heatsinks and maybe a chaff.