Its not impossible, I've defeated her by using medicham with detect and sturdy geodude lvl 1 with 11 hp. Basically you need a pokemon with detect/protect, one with sturdy, a full roster, couple of healing items and a revive just in case. Protect and detect have higher priority than decimation and they shield the pokemon against OHKO, swap in other team members to sponge decimation, keep the pokemon with sturdy in reserve until yvetal wastes all 10 pp and starts to struggle, then just heal it to full hp.
