For this achievement you can either wait or purposefully get to less than 50 health and use an ultimate, or you can follow this solution as it will unlock both achievements:
My solution for "From the Grave" recommends that you use the starting character Viktor to toss out a grenade (
) while both you and your opponent are very low on health. If they kill you while you're tossing it, and the grenade hits them, you will unlock both achievements for 75 GS.
Another useful method for both achievements is to play as Skye, run into a large group (especially near the objective), and use her ultimate which deals massive damage. You'll die and most likely unlock both achievements.
Credit for this method goes to Slayers Reign
Note that you must play against bots until your account level is 5 upon starting the game before playing against other players to unlock achievements. This usually takes 4-6 games depending on if you win and how well you play.