There is an FAQ here (http://db.gamefaqs.com/computer/doswin/file/master_of_orion.txt
) that has a by the numbers look at things.
Take a look at section 6.4 for the guardian's stats at various levels and how many small ships per beam type you would need to defeat it. Things get pretty nasty at the higher levels.
I have never had much luck taking it out with missiles due to its lightning shield. I usually use medium ships with my best beam weapon at lower tech levels. At higher tech levels I use larger ships stuffed full of all the good toys.