As mentioned in the other solutions, this achievment can be unlocked at the very end of the game
and is missable
if you get defeated. The difficulty does not matter.
If you approach this one on hard difficulty it is adivsed to upgrade your health to the maximum, Just to be sure. At this point you should be pretty familiar with the fighting mechanics of the game, thus this isn't too difficult. However, since missing that achievement would be extremely "inconvinient" I would take the following tips into account:
1. As mention, upgrade your health!
2. Upgrade your explosives, so you can place 3 explosives at once.
3. You don't need to worry about the guys in front of the corridor. Just focus on the guys in the corridor. 4. Before you start punching, place 3 explosives at different places on the walls. Let them explode and most of them will be unconscious before the fight starts, giving you the opportunity to take out 2 or 3 of them before you have to go into close-quarters combat.
5. Whenever you have the feeling that things could go south for you (e.g. you can only take one or two more punches) Pause the game, dashboard, start another game and go right back to Arkham. Dashboarding should only be your last resort, but better safe than sorry! You came to far to blow your completion on the last mile. *** As pointed out by another member here on TA this step seems to be obsolete. Here is what he has written: "step 5 is not needed and should be removed, because closing the game takes much more time. You can die and restart just before the battle party begins. ;o
Using this strategy should give you that achievement on hard difficulty in no time.
(video credit: Gyoru)
Hope this helps!
Any improvements? Please let me know in the comments below!