The vital statistics for the original Eternal Guardian:
You examine the eternal guardian. Your diagnosis: DV: 30, PV: 60, Hits: 1817.
As mentioned if the PC does not teleport to the stairs immediately, a duplicate will appear. The vital statistics for the first duplicate:
You examine the eternal guardian. Your diagnosis: DV: 30, PV: 60, Hits: 2507.
The trend is obvious. Fighting multiple copies of the Eternal Guardian is a recipe for suicide. Or a recipe for lots of experience, if the PC is fast enough. The Eternal Guardian is not especially fast, so hit and run tactics can be used. He is vulnerable to humanoid slayers or ball spells. It takes a lot of either to bring him down though.
Even if the PC manages to pass him without the Ring of the High Kings, the shortcut to the surface through Darkforge (see section 3.4) will not work in the down direction - a distinct disadvantage.
It is possible to anger the Eternal Guardian, kill his hostile copies and still pass peacefully by showing the Ring of the High Kings to the nonhostile copy that remains on the stairs. This can be used to exploit the Eternal Guardian for experience.