Charles-Valentin Alkan's '3 Improvisations dans le Style brillant' Op.12 are highly virtuosic and demanding pieces, characteristic of his challenging and original approach to piano writing. They require immense technical facility and stamina.
Address the extreme technical demands, focusing on developing rapid octaves, large chords, and sheer finger power.
3 Improvisations dans le Style brillant (Op.12) is rated Henle level 7 of 9 — late advanced repertoire, roughly ABRSM grade 8 and above. In the RCM system it corresponds to about level 10.
You should be comfortable at Henle level 7 — meaning you can already play level 6–7 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of 3 Improvisations dans le Style brillant (Op.12) lasts about 15 minutes.
3 Improvisations dans le Style brillant (Op.12) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.