Hamilton's Great Adventure Demo

Getting Achievements in the Demo
To get these achievements to pop retroactively, we can use our main game's save. You can do this by creating a .txt in the demo's folder (which should have all of the base game's files instead of the demo ones) called "steam_appid.txt" and typing "42190" into it.

Retroactive Pops
If you had all gold ratings on your base game save, the achievements this guide is tied to appear to retroactively pop after simply completing a level. For my exact retroactive unlock conditions: I got a Gold rating on Tomb of Osiris, which was a level already accessible due to my base game save being 100%.

Easy Level Select Achievements (with 100% save)
Now, for most of the other achievements, you can easily get them by going into their applicable levels via level select. There are other guides online (such as this one: https://www.playstationtrophies.org/forum/topic/111502-hamiltons-great-adventure-road-map-and-trophy-guide/) that can tell you what they demand. Some of these just popped up during normal gameplay in the base game, so I didn't know the exact criteria that made them pop; that information, as well as guides on King of Bling, is easily available through the link above.

A note on Egyptologist: this does NOT appear to be obtainable via level select if you've already gotten it on your 100% save. This could possibly be due to the game thinking you already have this achievement.

Note that the save functionality is still broken when launching hamilton.exe through the demo appid on Steam.

Achievements with Fresh Save
For the next section of this guide, I want to go over the achievements that do NOT pop retroactively and that do NOT have a way to obtain via level select. Mainly those are the "Complete the X chapter" achievements, which only pop when completing the final level of a chapter for the first time. I got many of these by playing through the game on a fresh save, but given the game's own faults, you are bound to crash and be unable to complete all of these in one session. Luckily, you won't need to get gold ratings along your way, though, since you should already have those from retroactive pops.

I have yet to test this, but it may be possible to import saves from the base game that have partial completion (such as every level up to and excluding the final level of chapter 4) to gain these kinds of achievements without having to risk a game crash after hours of gameplay. The same goes for Egyptologist. I will update this guide when I am able to test this, but I see no reason why it shouldn't work.