It can take away the PI.
Amp damage reduces physical resistance by 100% (Decrepify reduces by 50%).
Monsters are "immune" if their resistance is >=100%. If that's the case, Amp/Decrep only works at 1/5 efficiency (as far as I know), so amp takes away 20%, decrepify takes away 10%.
Therefore, if the physical resist is less than 120%, amp will remove 20% and the monster will be below 100% --- no longer immune.
If the monster had let's say 130% physical resist, amp will take it to 110%, so it will still be immune.
20% doesn't sound like a lot, but it seems most PI monsters aren't even close to 120% --- 119%-20% would leave it at 99%, but I can kill amped PIs failry quickly, usually; so I would suspect that they only had 100% to start with.
EDIT: Lower Resist works the same way for the elemental resists.