This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
蔪 combines 艹 (grass) with 斬, but the historical development and original meaning are uncertain.
The grass radical 艹 sits above 斬, but without a clear meaning, the visual shape is the main memory aid.
For セン, imagine a thousand (千, せん) blades of grass being cut by the axe-like 斬 below: 千 -> セン.