"Save players from death 250 times"
What are the mechanics? What methods can I use?
According to this website a save is when you
granted the other player is reasonably close (my guess here is within 1 screen).
This answer is up to date as of 05/18/2014, game version 2.0.5, and comes from having farmed the achievement right now.
Getting a Savior callout requires:
The "victim" player should have two sets of items: one with maximum Life, another with no Vitality at all. Swapping to and from no-vit items brings your health down, though there is some kind of threshold, and your health settles a bit higher than your "naked" health.
Actual farming (we did this in Old Tristram Road with zombies, it is also doable with Carrion Nests and endless bats):
Every now and then one of the players will inadvertently step on a health globe. The victim has to stay below 5% life, so he should run out and take some hits. On the other hand, dropping too low is also dangerous - if you die and are revived, you'll be at full life and will have to swap again. Ideally, the victim should have a controllable regen gear or healing spells to stay at 4-5%.
If you survive an encounter with less than 5% of your health, you get a "Survived" fanfare (along with a % of your HP that remains).
Saving players would involve killing the last creature in combat whilst a party member is on low HP. Then do this another 249 times.