critical/deadly and amp damage I know critical/deadly doubles final damage. Does amp damage do the same thing? and what about if you cast an amp damage and then land a critical/deadly strike?

Critical/Deadly and amplify damage work in two different ways. Amplify damage lowers the enemies physical resistance, hence why it can break Physical Immune. Critical/Deadly simply doubles damage done. This means that an amplified enemy normally at 0% physical resistance is taken to -100% for double damage, then critical/deadly double damage when the random number generator rolls the right range. An enemy normally with 50% physical resistance is taken to -50% physical resistance from amplify damage, meaning they effectively take three times more physical damage than normal. Critical/Deadly doubles that damage when rolled. An enemy normally with 100% physical resistance is taken to 0% physical resistance from amplify damage, meaning they effectively take infinite times more physical damage then normal (whatever your actual physical damage is). Critical/Deadly doubles that damage when rolled. I'm about 99.9% sure of this. If anyone wants to confirm or deny, please do.

I believe things that break immunities (which includes Amp) only work at 1/5 effectiveness against immunes, so a monster with 100% physical resists will only get reduced down to 80% with Amplify Damage.

amplify actually quadruples your dmg when you do a critical hit - since criticals will now do 4x dmg instead of the usual 2x very powerful stuff, my zon actually does 18,000 dmg multi (78% chance) when i'm teamed up with a good amp necro poor hammerdins who are forced to look at this mass destruction in cow/cs/baal runs

so when im dueling another player, and i cast amp damage on them, then fire a a guided arrow at them and score a deadly strike, how much damage will i do? lets assume the bow attack gets 4k damage and they have 50% dr. (is dr like pyshical res?)

Sorry I can't help. The forums are crap and I keep having to log back in to receive an 'Invalid Thread' response. Note to owner: FIX IT.

PvP damage is /6, so to start with, 4k damage becomes 666.666666...... If they start with 50% dr, and you amp them, they now have -50%. This means an increase of 50%, so multiply 666 x 1.5, for 999 - taking those extra .6's into infinity into account, we'll call this 1,000. 1,000 damage doubled from the critical/deadly strike is 2k - so that's your final PvP damage in this situation.

1920. PvP damage is .16*damage, not damage/6. Also, the formula order is off. 4k damage should be doubled first, then cut to .16 of it's value, then times 1.5. In this case, the order of operations didn't matter, but it sometimes might.

Rule of thumb #1: do the calculation in the worst possible order. IOW, put yourself in the position of the defender and do the damage formula, then in place of the attacker and do the damage prevention. Rule of thumb #2: think like a programmer, i.e. only figure out the damage if a successful hit is registered. This keeps the processor from having to calculate damage every time you swing, instead just when it's actually going to matter, etc.