There is no 1.0 floor for mob cRatio; I tested. However mobs get positive level correction when fighting players, which means it's virtually impossible to get cRatio that low when fighting high level mobs.

Ig-Alima (lvl 120) hitting a pld who has 1000 defense would be working with a 2.06 cRatio (or maybe 2.11; little fuzzy there). If the pld miraculously gained another 1000 defense (2000 total), Ig-Alima would still have a 1.53 (or 1.58) cRatio against him.

Also, the claim that 1500-2000 is common for strong NMs seems unsubstantiated. The only known attack value we have is 1059 for Ig-Alima.