ecounter level and such
hopefully this will help, should have posted earlier but I figured someone else would.. anyway
dmg page 100 challenge ratings and encounter levels
this is pretty close to one of the arguments, it says here that challenge ratings are based on parties of four characters, I know it says it elsewhere, but this one quote should fix that problem
it also says that groups of more characters would take on equal challenge ratings much easier
on page 101 of the dmg there is a chart, lets come from the point of view of the happy go lucky monk guarding the whatever place shall we?
monk comes across the party, it averages out as basically a 6th level part ( 2 x 5, 2 x 6, 1 x 7) (*imho +8 str, +4 con, +4 dex, +2 natural, increased move, resistance to a lot of special attacks would be +3 levels, then again, +8 str is pretty much equal to +2)
anyway though, tangent
going with the chart he sees 5 characters roughly 6th level average.. moving over this encounter is roughly EL 11.. so, if he had three other people just like him to make a 7th level party they would still be under by 4 levels.. in other words, overpowering for a party of himself, and EXTREMELY! impossible for one.. any non-spellcasting character who did this battle would be lucky to get through 2 rounds, max.. the spellcasters have more tricks so are harder to judge
so just from this it seems that he doesnt have a chance to scare them off, nor would said barbarian, fighter, paladin, ranger, rogue.. whatever, it is just too far above there level
now, if you are able to change terrain to his own purposes then yes, anyone could beat this party off, even a single level 1 kobold.. that isnt the point
unless you take drastic measures for terrain he will have no chance, no non-spellcaster would, the challenge before them is a good challenge (potentially have to run away) for your group vs four 7th level npcs.. putting pc gear on him helps a little, but not even enough to make him count as two people, let alone 4
hope that helps ;/ its not a question of how much damage will be done, its a question of whether it will even take 2 rounds to completely decimate said 'guardian' without changing the entire environment to suit this 'guardian'