I always read that passage as a literal log with eyes. Gollum hanging onto a floating log with his eyes just above the log in an effort to remain unseen.
It was an act, because he's supposed to limit himself to what mortals can do. Right afterward he fought with the Balrog(Balor) for 3 days and 2 nights before killing it solo.
He also cast several lightning bolts in The Hobbit, and cast many fireballs at Weathertop when fighting the Nazgul...
The dwarves also lost Ered Mithrin in the north to a cold dragon..
The elves forged a number of lesser rings that were not tainted by Sauron. The one worn by Saruman would almost surely have been a lesser ring, as Sauron would not have allowed a ring of power to be given to another Maia who...
Getting advantage two rounds in a row is very easy. Not guaranteed, but very easy. Getting bonuses to hit is also fairly easy. Not as easy as getting advantage, but still pretty easy starting at low levels.
I think 5e differs from prior WotC editions in that PCs can and do nova for craptons of damage, so a lower hit chance at mid and high levels will mitigate that a bit and instead of blasting things 10 CR over your level out of the water, maybe you just blast 3 or 4 CR over your level out of the...
I'm not sure this is all true. Not that the math isn't what you say it is, but rather that it doesn't take into account the various ways to get bonuses to hit, how trivially easy it is to get advantage which amounts to up to the equivalent of +5, etc. It also seems to assume 65% chance to hit...
I edited my post with the info from the 5.5e stuff. It should work fine for you. And I got the magic items wrong. LOL. 1 common and 1 uncommon. I listed the level 11-16 range in my last post.
Are you using 5e or 5.5e? I think the 5.5e DMG has more in it about starting equipment for PCs who start at higher level, which would aid with your NPC.
Edit: My 5.5e DMG says it's in the 5.5e PHB.
Edit 2: The 5.5e PHB says that at level 5 give 500gp + 1d10x25 GP, plus normal starting...