。
The key to raising every soldier into a warrior worth a thousand men lies in intensifying training.
Literal
All [genitive-の] soldiers [object-を] one-horseman-facing-thousand [genitive-の] warrior [into-に] raise key [topic-は] training [object-を] intensifying [thing-こと] is.
A dramatic aphorism. 一騎当千 (いっきとうせん) is a 四字熟語 literally 'one mounted warrior equal to a thousand,' meaning 'a single warrior who can match a thousand opponents' — an idiom for extraordinary individual combat prowess. Borrowed from classical Chinese military literature. つわもの (兵) is a slightly archaic/literary word for 'warrior,' 'brave soldier' — more poetic than 兵士 or 戦士. ~のカギは~ことである is a rhetorical frame: 'the key to X is Y,' identifying the central factor in some achievement. 強化する ('to intensify, strengthen') is a formal verb common in military, administrative, and industrial contexts.