OK, as amp isn't mantioned, I will assume that it's about a summon necromancer with skeletons and perhaps revives (plus a clay golem and a merc, of course). These guys are pretty good at solving the game solo, even with suboptimal gear, although there are a few obstacles which can be quite tough at the level they are typically encountered at, like normal Diablo. I usually run through 3-4 complete skeleton armies to defeat him with such a character. As soon as decrepify is available, act bosses aren't that much of a threat anymore.
Amp reduces physical resistance by 100%, so it works particularly well against monsters with high physical resistance, as the difference between 90% and -10% is a factor of 11, for example. For a monster with 0%, it will double the damage. Amp only affects physical damage.
20-fold returned damage isn't enough for hell mode. For example, a venom lord from act 4 has 9-15k life and his melee attack deals 90-150 life, i.e. 1/100 of that and for each player beyond the first, they will have 50% additional life while their damage increases by just 1/16 per player. Also, the better you are protected, the less the effect will be because the monsters will hit you less often, although I guess it's mainly about applying thorns through skeletons. Anyway, the stronger the minions are, the less they will get hit and the less damage will be returned. That doesn't sound like a good strategy.
Google