Here is the sentence I have seen them from a boxing Manga.
ひじを左{ひだり}わきからはなさない心{こころ}がまえで..... やや内{ない}角{かく}をねらい
えぐりこむようにして...打{う}つべし!
Especially 「べし」, I don't know the purpose to use it in the context.
Another question: As my understanding to the content, am I correctly translating?
Translation: From the left armpit, keep the elbow close to the front of the chest and aim to the inside corner (of the opponent) then move forward (to him) to punch/hit.