Shielding (aka "smothering" isn't as simple as you like to make it out to be and I don't think it has a whole ton to do with strength.
We've seen several channelers fight off shielding. Moghedien and Nynaeve's duel was the most obvious example of this
We know skill plays a solid role in it from Graendal being able to cut off and shield Aran'gar despite a good amount of strength difference in Aran'gar's favor. Siuan, possibly 8 levels lower, was also able to cut off and shield Nynaeve back TGH, and even told her with time and skill no one Aes Sedai would be able to do so.
We know there is a least a minor Talent involved from Berowyn's shielding of Nynaeve
We know that even a significantly stronger channeler can struggle to shield a weaker one (Nynaeve could not shield Elayne who is at least 6/7 levels lower than she is at that time). But with a more skilled channeler it can be done rather quickly, Egwene easily cut off and shielded Sheriam who was 6 levels below her.
Honestly, other than a minor role in shielding, strength doesn't really play a significant role in a battle situation. Daigian, left unshielded, can still kill any of the Forsaken if she's smart about how she attacks. Defensive capabilities are seen from Demandred, Graendal and Aviendha at the least, speaking to skill playing a larger role in a duel than strength.
Cadsuane had to know that she was at a skill disadvantage at the Cleansing and wanted to ensure that her people at least had some level of strength advantage if she could manage that. I do find it a bad choice to leave Merise and Narishma essentially sidelined for the battle though.