Re: Is there a much easier way?
With this question, you are entering the wonderful world of Magic Finding (MF) :thumbsup:
I'm sorry, but there's no way to quickly get the particular sets or anything you're looking for. But luckily there are a few things to make sure your odds are at least a bit better.
In short, you just try to wear equipment that gives "better chance of getting magic items" as much as possible, while still maintaining good killing speed. Usually it's good to have like 200-300, but if you're just starting, don't worry about it. Then you go and kill boss monsters over and over again (really, you're looking at hundreds, if not thousands, of runs to get the really good stuff). In your case, Mephisto or Andariel is your best bet. If you feel it's too dangerous, get a few levels for you and your merc. It'll get easier with levels, better gear and most of all, playing experience.
I don't know what you mean by how long does it take until you can kill him again? After killing him, just start a new game and kill him again.
Also, if you are able to run NM Mephisto, the Milabrega and Sigon sets will soon be collecting dust in the darkest corner of Larzuk's shop... :wink: He can drop FAR better stuff.
If you're interested to learn more, there's a wonderful guide in the stickies by Hrus:
MF Guide for 1.10/1.11
And, if you don't think endless grinding for better gear is your thing, read up on jiansonz's tips:
Guide to Untwinked, Single-pass, early game Magic Finding