常人なら一生かかっても無理だろうが、本気になった蓮の前では、普通のシリンダー錠と大差ない。

、、。

An ordinary person would never crack it in a lifetime, but for Ren when he's serious, it's no different from an ordinary cylinder lock.

Literal

Normal-person [if-なら] lifetime take-[even-ても] impossible [probably-だろう][but-が], serious-became Ren [possessive-の] before [in-では], ordinary cylinder-lock [with-と] big-difference not-exist.

常人 'an ordinary person.' 一生かかっても無理 'impossible even if it takes a lifetime.' 本気になった蓮 'Ren when he gets serious' — a common character-reveal pattern in fiction. シリンダー錠 'cylinder lock.' 大差ない 'no big difference.' The sentence hypes a character's extraordinary skill.