Aに言わせれば=In A's opinion (If you make A speak...)

さておき周りの風景が色違いになるほどに月は動揺しますが、Lに言わせれば神の域に達しているらしい精神力でどうにか抑え付けます。
Light was shaking, but in L's opinion (he would say) Light he's supresing his supresing his emotions because of his god-like emotional strenght