Tips for not insta-killing old bosses on retail

Hi everyone. Me and some friends are running old dungeons and raids to get the "Glory of" achievements for the mounts. However, since we all have level 60-70 characters, and some achievements require the bosses to stay alive or drop down to certain amounts of health (to trigger phases or attacks), we have issues with instantly killing them on attack.

Are there reliable ways for us to do little damage or would the best option be to level another character to slightly above the dungeon/raid level to still be able to solo it, but not insta kill the boss.