Sometimes, yes. Amplify damage reduces enemy physical resistances by 100% if they are not immune. At 100% resist and higher (immunity), AD works at 20% efficiency. It's impossible to break a 120% physical resistance.
Monster with 0% PR. AD takes 100% off, reducing PR to -100% (resistances can never be lower than this)
Monster with 99% PR. AD takes 100% off, reducing PR to -1%.
Monster with 100% PR (immune). AD works at 20% efficiency, so 20% is taken off, reducing PR to 80% (immunity broken).
Monster with 120% PR (immune). AD works at 20% efficiency, so 20% is taken off, reducing PR to 100% (still immune).
Decrepify reduces monster PR by 50%, so 110% PR is immunity to Decrepify. Both AD and Decrepify are curses, so they cannot stack.
Conviction (Lightning Cold Fire) and Lower Resist (Lightning Cold Fire Poison) do stack, though they suffer the same reduced efficiency against immunities. 144% resistance against Lightning, Cold or Fire cannot be broken, nor 114% resist against Poison.
Keep in mind, players can't be immune, so AD, Decrepify, Conviction and Lower Resist work at 100% efficiency against them. I'm not sure how this applies to summons with immunities (such as the Druid spirits on NM), but I assume it's the same as monsters.
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