« être battu comme plâtre » is fine for situations such as fist fighting. In the case of flogging one can use “fouetter jusqu’au sang”. The register of those expressions is however current. There does not exist, apparently, a phrase that would include the word “merde”.

In the context of an abuse, for the description of a one time event,

• you could use:

  Brother Luke m’a foutu une (putain de) raclée.

  Note that foutu is already vulgar language, probably as much as shit is, but you can add putain de if you’d like to insist on that aspect.

• or at the expense of a significant language register mismatch you could use:

  Il ne m’a pas raté.

  It probably fits better if it’s used as some kind of a “punchline” (pun not intended).

In the case of a general description of repeated events, the first one can be modified slightly like so:

Il me mettait des putains de raclées.

Note that I chose to discard “Il me foutait” because it lacks solemnity.

Just in case, any other familiar expression I thought of before knowing the context would forbid sexual violence connotations (if there was any) and most of them wouldn’t be quite suited to describe “physical abuse” by a person in charge in any case anyway. I guess there are more but they didn’t come to mind yet.

I would suggest:

Je me suis pris des putain de branlées par frère Luke.