Most likely to get first.
On the first level, open the hospital storage room after entering cyberspace for the first time.

During level 4 exploration you 'skated' (it's recommended to activate the "Turbo Motion Booster System" before the jump) down the ramp in Storage

Access to the maintenance office was gained at level 3. To achieve this, it is necessary to advance the plot until the security level allows reaching the retina scanner. To open this door, it is necessary to carry the severed head (shown in the image) and use it on the scanner.

Completely reduce the security level in the Citadel to 0%.
This is achieved by destroying each of the surveillance cameras, as well as the security nodes (and not forgetting to have dropped the vegetation capsules as part of the story).

Complete the game.
That completing a 1994 game (when a remake is available) is an achievement in itself these days, right?

Finish the game on all maximum difficulties.
The most difficult of achievements. This is configured at the start of a game by selecting 'level 3' in each game category (Combat, Mission, Puzzle, Cyber). Due to the time limit of 6 hours to conclude the game , it is recommended to familiarize yourself with the map beforehand, or to look for a quick guide to achieve it.

Update: Some users have commented that the time limit is an exaggeration. It is enough to finish the game on the highest difficulty in 7 hours to unlock this achievement.
This is not information that I have personally verified, but it is worth mentioning.