I figured I'd post this since I was myself genuinely baffled and frustrated when I first saw this boss, and on some other forums I saw people complaining about not being able to do it on Sniper Elite difficulty on solo.
First of all, the boss has skulls flying around him. You need to shoot them all and destroy them, then he'll be vulnerable. I don't know if his entire body is vulnerable, but I just kept shooting him in the head. Secondly, while you're trying to deal with the skulls, he also summons a mob of enemies to chase after you. If you kill all the enemies he'll just summon more, so there's no point in killing all of them. I've heard that you can leave two and he won't summon more, so at least just do that.
The trick is to just keep moving so the enemy mob can't swarm you, and take chances to shoot the skulls and then shoot at the general's head. You fight in a church where there's two floors, and you need to keep moving in a circuit from the lower level to the higher level and back again. Also there's ammo and explosives on the upper level.
Basically what I did was:
- Run from the mob and get them to start slowly chasing me - Change floors - Stop in an open area (either the bridge on the upper floor or the open area by the boss on the lower floor) and unload SMG fire on the boss's skulls. Meanwhile I make sure to look left and right to see if the mob is near me. If the mob is near me and the skulls aren't all destroyed, I run away and up/down the stairs to the other floor. - Then I unload smg fire on the skulls to finish them off, then whip out my sniper rifle, lay down on the ground, and fire at the general's head. Sometimes the mob would swarm me and hit me a few times but I'd manage to run away.
I brought an SVT to the battle, which has a 10 bullet clip, and I'd manage to get about 9 or 10 bullets to his head before he became invincible again. Then his skulls return, and I'd keep moving and rinse and repeat. After destroying the skulls twice and filling his head with bullets (about 18 to 20 bullets I guess in total) he'd die. BUT he has three forms so don't get excited after you see him die the first time. Fortunately you do get a checkpoint save in between each round.
- First form: 3 skulls, and he summons a zombie mob. Fortunately the zombies are slow. - 2nd form: 5 skulls, and he summons a skeleton mob. The skeletons are way faster but I found that by constantly moving they rarely caught up to me even when I'd lay down and fire 10 bullets at the boss. Probably it would be a good idea to kill some of them to lessen their threat. Pace yourself as you shoot at the skulls so you don't make him drop invincibiility right when you have to run away and waste time. - 3rd form: 7 skulls I think, and he summons a zombie mob but also one of those annoying uber zombies with an MG. I tried killing the MG guy but I ended up just running away from him instead and killed the general boss same as the other rounds.
Hopefully this makes sense, I know it's long. Again basically you just have to avoid the mob and shoot the skulls and cause his invincibility to drop, then shoot him. Again the main trick for me was to just keep moving. Overall definitely a tense and tough battle.
Great game too, it's a little rough around the edges but it's survival horror done right, which is rare these days. --- It is bad when one thing becomes two. One should not look for anything else in the Way of the Samurai.