In Dark Souls I love them, in Metal Gear Rising I hate them. In Dark Souls I feel like it’s my fault, in MGR I feel they’re nigh unavoidable.
Good idea is to have auditory cues for offscreen enemies, or some other type of attack indicator. I prefer audio, we can do 3d sound fairly well these days, and audio I think can be a bit more clear about the pacing of the attack, and I feel like it’s the “right” level of feedback clarity directionally.
The big thing is, players need a fair chance to perceive that they might be threatened from offscreen. In Dark Souls, this usually only happens if you rush past enemies and let them surround you. Enemies are not easily missed in Dark Souls. When I am engaging multiple enemies, I know there are multiple enemies and I keep a mental map of where all the enemies are. I really enjoy fighting multiple enemies at once because the game makes it possible. In MGR, the camera is practically biased to force you into walls, so enemies are always coming from offscreen until you are down to 1 enemy and can finally camera lock them without ignoring the rest. Many enemies have ranged attacks that need to be blocked up, or they’ll dash into you as they attack. Dark Souls has no soft-lock, and the jumping attack and kick commands can be done in any direction. MGR has a soft lock, so if you do the stinger move or the wide slicing trip move, you won’t hit inbetween the two/four enemies where you aimed it, you’ll hit one or the other/three of them but not the fourth.
Cheating a bit and altering enemy aggro/attack speed when offscreen is perfectly permissible in my book. Just don’t do something silly like Amnesia where the monster practically can’t hurt you if you look in the right direction and relax those restrictions if the player engages in combat with one of them, because then they’re asking to get smacked.
God Hand gives the player a Radar, given the camera is like a tank and mostly fixed, this is probably necessary for that game, it seems like UI clutter or overkill for many others. God Hand AI cheats like I mentioned, except on LvDIE.
The idea is, either make sure the players know about all the enemies around them before they turn their back, or give them a warning they can see or hear before an unknown offscreen enemy attacks in a way that gives them a chance to react correctly. Or both.