Since some players have already defeated the nether bosses at certain nether realm levels the achievements are impossible to obtain without another save file reaching them.
Is it possible to provide the achievement if the player defeats the nether boss or visits the nether realm and clears it.
Player already has defeated nether realm 5 boss.
Player reenters nether realm 5 and progresses to realm 6 through “statue/crystal”.
Player receives the achievement since they cleared realm 5.