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i know a better guide will probably be made but here goes any way. what we know for this is yes new game does carry over your level all combo weapons safehouse upgrades etc. I suggest being at least over level 70. zombies hurt more at times and weapons break a little bit faster than normal headshots are a must as well. beating the game on this difficulty should net you both achievements and if someone can confirm this i will update the guide. also use combo vehicles when out side they will help alot my suggestion use the sling rot. happy hunting guys and please any comments that can help this guide out will be appreciated and credit will be given to those when i update it
edit: a couple of tips during the Fontana fight shotguns and the explosive tubs were my friend just let her get near one and shot the tubs twice she gets hurt bad and gives you a few extra shots in before she regroups herself. during the calder fight just make sure your stocked with food and use the presents that go on your fists until you get the arcade power upgrade. the last part when you have to get to the roof try to have a tri pod zapper with you and set it done one at a time they do wonders.beating the game on the blackest Friday difficulty will net both achievements
credit: credit goes to SWDY1 for letting me know in the comments level 100=no problem on this difficulty and also combined your weapons with golds bars as well because like he mentioned they don't break as fast. and credit to ClawColyer for mentioning that food doesn't heal you as much and too always have a full stock. happy hunting everyone
credit2: quick shout out to moldredd for posting extra helpful tips in the comments be sure to check the comments out for that info.