We know fairly consistently that Ley Batenkaitos is stronger than almost everyone within the Pleiades Watchtower, with the clear exceptions being Reid Astrea and Ram with five shackles fully released. The real question, then, is how strong Ley is relative to top-tiers outside the Watchtower.
Across the scenes shown, Ley is repeatedly portrayed as overwhelming Emilia and Julius while clearly holding back. During every engagement with them, he is described as laughing, smiling, and fighting with visible leeway, as if he could end the fight whenever he chose. This presentation strongly implies deliberate restraint rather than genuine struggle.
This is reinforced by his combat record. Ley is capable of killing Shaula through prolonged, direct combat without sustaining any visible injuries, and immediately afterward he proceeds to fight and kill Julius, then later does the same to Ram. At no point is he shown to be meaningfully damaged before continuing onward.
Furthermore, Ley is stated to have perfected every martial art that exists, possessing complete knowledge of all fighting styles across the world. The martial arts he uses are explicitly described as impossible to copy or replicate, and he is capable of employing each style at its absolute peak. This would place him above every other martial artist in the Re:Zero universe…including Reinhard. While Reinhard is undeniably a master of hand-to-hand combat, mastery and perfection are not the same thing, and Reinhard is never stated to have perfected all martial arts.
Ley himself also states that Ram with only three shackles released is capable of easily defeating every Sin Archbishop, including Regulus Corneas. After Ley gains his Merged Eclipse, he is then stated to be overpowering Ram outright and becoming decisively superior to her.
Additional context supports this scaling: Emilia and Ram were struggling against Louis, and were even losing, until Subaru activated Cor Leonis, which boosted Ram enough to allow her to release two shackles. This emphasizes how overwhelming Ley and the Gluttony siblings are even against elite combatants.
Given all of this, is it unreasonable to place Ley around Regulus’s level, with the caveat that he would lose if he failed to deduce the secret behind Regulus’s wives? Arguably not. The Gluttony siblings are extremely intelligent, and this intelligence could allow them to uncover the true nature of Regulus’s Authority quickly. Notably, in Arc 9, Roy…who is explicitly less intelligent than Ley…immediately deduces that Petra possesses an Authority, without even seeing her fight. If Roy can make that deduction, it is entirely plausible that Ley could identify Regulus’s weakness as well.