Building on the answers from Max Williams & Jasper Locke I continue to make the case for on.
Even though generally the preposition in is more common with mode than on. Ngram
Also more specifically "in adjective mode" results in tens of thousands of hits on Google Books Corpora while "on adjective mode" results in 162 total.
If searching for play on/in hard mode the difference is bigger than in Max' search:
33k on vs 12k in
It's not uniform through other difficulty settings though:
- play on/in easy mode (20k on vs 9k in)
- play on/in normal mode (17k on vs 40k in)
- play on/in beginner mode (2.7k on vs "did you mean "play on beginner mode")
In a nutshell, generally in is used in conjunction with mode and gamers can't seem to decide which one to use. It even varies between difficulty modes.
Yet you also ask, Is there a proper way to say that in a more direct fashion?
I would simply suggest to use difficulty (level) instead of mode. Indicators from google searches (275k to 35k in favor of on) to the random sample I did of steam achievement texts (Beat the game on any difficulty level, Overthrow the aliens on Legend difficulty) seem to suggest on is more popular.
Furthermore on works nicely for all extensions or shortenings of the usual sentences and can be used well with both difficulty and mode:
- Play on hard (Even excluding mode or difficulty the results are almost purely gaming related, for play in hard, gaming takes a clear backseat)
- Play on hard mode
- Play on hard difficulty
- Play on hard difficulty level