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Here's the stat of Leth Tyrant ring:

Critical Damage: 15%
Sword amp: 2%
Magic amp: 2%
HP: +10
Defence: +10
attack: +5
Magic: +5

Because the maximum crit rate you can get from rings is 30% so Merg + RoL1 + RoCham4 is enough, and you also have some little magic from Ring of Champion. Compare that to Merg and 2x RoL1: you waste...