EDIT#2: This achievement along with at least one other may have been broken by the most recent update. I will update as soon as it seems it's working again.
hit the ~ key and type "map e1m7" and finish the level. You need to pick up the rune in front of you. Turn on "god" mode (console command). As long as you pick up the rune, you can turn on "noclip" and go straight to the portal this time, no need to kill the boss. Jump through the portal for your achievement.
(please note level select seems to nullify some achievements, but using the command code work with no problems)
***EDIT: It seems GOD and NOCLIP have been disabled in a patch***
IF GOD MODE ISNT WORKING: (all credit goes to Chocotoaco119)
According to a recent post on reddit you need to type "sv_cheats 1" hit enter and then type "god". You should get a notif in the console that god mode is on.
MAKE SURE YOU TYPE "sv_cheats 0" AFTER YOU HAVE ACTIVATED GOD MODE OTHERWISE YOU WILL NULL THE ACHIEVEMENT. God mode will still stay active after entering