Amplify Damage

This is typically considered a "1 point wonder", in that additional points only increase radius, and not the amount of damage amplification. Therefore, every necromancer build only puts one point into it, and any additional points are provided by +skill gear and charms.

Damage Reduced by % is capped at 50%, but you can stack it to compensate the loss from

Decrepify or

Amplify Damage curses.

Every necro build needs this curse. That one skill point works wonders for this, same with

Corpse Explosion. I've run no/low curse skill point builds, but I never skimp at least +1 on amplify at least. Big payoff for such a small skill point investment. +Skill items buff it by increasing cast radius.

In Diablo 2 Monster Physical Resistance is capped at 100%.

Curses work at 1/5 efficiency when breaking immunity. That means:

1. All monster's Physical Immunities can be broken by
   Amplify Damage.
2. The resulting Physical Resistance of said monsters will be exactly 80%

The

Sanctuary Aura sets the Physical Resistance of monsters to 0.

Amplify Damage is apllied before Sanctuary so when using the latter it is pointless to use

Amplify Damage.

With the physical sunder charm

Bone Break in your inventory, both

Amplify Damage and

Decrepify work at 100% effectiveness.
This is unlike how elemental damage sunder charms interact with

Lower Resist and

Conviction, which only operate at 20% effectiveness on sundered enemies.

An addendum to the comment above - monster physical damage resistance will never naturally be higher than 100%.

However, adding the Stone Skin mod CAN increase this percentage to over 100%.

This becomes apparent if, for example, you run into an elite pack of Blood Lords with the Stone Skin mod, as you may notice that Decrep or Amp does not remove their physical damage immunity in such cases. Because both these curses work at 1/5th effectiveness without the use of

Bone Break, in such cases only reducing their physical resistance by 10 or 20% respectively still leaves them at >100%. This also applies to variants whose physical resist is 75% buffed by Stone Skin up to 125%.

This is uncommon as there aren't a ton of monsters in the game whose base physical resist is higher than 50%, but there are a handful and this can be a problematic when it happens.