What is a single word to describe "beating someone brutally"?
"Thrash" is a good word, and looking at a thesaurus gives many others including batter, clobber, maul, paste, pound, pummel/pommel, wallop, assault, bash, drub, etc. Meanings depend on context and some of these can be informal/mild/humorous; before using a word, look up its meaning and examples of usage.
Bonus Indian-English slang word: thulp, probably derived from "thrash/thump to a pulp". :-)
Depending on the weapon used, you might try fustigate, meaning to beat with a cudgel; this would be particularly appropriate if you wish to convey a certain archaic tone.
Some words you can use:
M-w.com gives the following definition for the word:
" to beat with or as if with a rod or whip."
Again, referring to m-w.com yields the following definitions:
transitive verb 1 a : to thrash soundly : b : to beat by a wide margin : 2 : to hit with force :
lambaste, trounce and sock are referenced as synonyms.