If you don't know about Tunnels & Trolls' "Death Spiral", this is it in a nutshell. As a monster takes damage those numbers come off it's Monster Rating or MR. Damage dice and adds are directly determined by MR (MR doubles as HP in D&D terms. MR is recalculated at the start of each round, so as a monster takes damage, it's ability to deal damage also goes down - therefore the "Death Spiral."
As I've said elsewhere, converting to another system is more art and less science - you go for the feel, not the reproduction.
For Swords & Wizardry, you could replicated such an effect by making it "1" harder for the monster to hit for every 5 points of damage taken as well as make it "1" easier to hit them for every 10 points of damage taken. I think something like this would be more appropriate in a Swords & Sorcery styled campaign, where heroes are expected to be heroic and perhaps in sessions with fewer PCs, as it helps change the balance as an encounter goes on.
Far Away Land has a much smaller power curve than S&W or T&T, so I would go with a lighter touch - when a monster has taken half it's hits in damage, remove 1 die from Brute / Str for purposes of attacks. At 1/4 hits, remove a second die. At no point can a monster go below 1 die.
MIDDLE DUNGEONS LEVEL FIVE 146 SHADOWY ROOM. - 146 SHADOWY ROOM. A dim blue-green glow from the south wall pierces the unnatural darkness of this room. It can be seen even from the doorway. Six shadows ...
38 minutes ago