I got the achievement for “Normal Completion” which surprised me a little because I only beat the last part of the game on Normal.
The next time I clicked “continue” with the intent to beat the final battle on “Hard”, and it just went to the final cut-scene and I got the achievement for “Hard Completion”!
I tried it on “Casual” and got another achievement. It seems the only difficulty this wouldn’t work on was “Hard Boiled”, but I’m certainly not complaining. I’m not sure if this works if you beat the majority of the game on lower difficulty levels, but I just wanted to share this with the world.
If you load that last checkpoint via Continue game, you can get the ‘Old School’ achievement as well!
Kill the 3 guys that come out of the door to the left of the stairs to spawn the ones who comes out from on the stairs, then just run up the banister (or just run up the stairs like normal) shooting all the signs and vents above them until you get to the top.
It might take a few goes to string it all together but if you do it right you can get a combo of over 50+ style points no problem (my top record on that area so far is 79).