Another fairly literal translation - The Orthodox New Testament - suggests the surpassing greatness of his power. Both ἰσχύς and and κράτος are in the genitive (as you probably noted), so the phrase could be rendered either as "the might of his power" or "the power of his might".

ἰσχύς is not very common in the New Testament, appearing only 10 times. It is related to the verb ἰσχύω which means to be able or have power. κράτος is just as common, appearing only 12 times. It is related to the verb κρατέω, which means to seize or arrest.

ἰσχύς is much more common in the Septuagint, where it appears over 300 times; whereas κράτος appears around 50 times. The two appear together in Deuteronomy 8:17, where Brenton proposes translations of "strength" and "power", respectively.

μὴ εἴπῃς ἐν τῇ καρδίᾳ σου Ἡ ἰσχύς μου καὶ τὸ κράτος τῆς χειρός μου ἐποίησέν μοι τὴν δύναμιν τὴν μεγάλην ταύτην

 

Lest thou shouldest say in thine heart, My strength, and the power of mine hand have wrought for me this great wealth.

So we might think of ἰσχύς as the quality or state of being strong and κράτος as an ability to act (following roughly COED definitions). But the two are so closely related - in English and in Greek - that you might be right in discerning a pleonasm.

Another fairly literal translation - The Orthodox New Testament - suggests the surpassing greatness of his power. Both ἰσχύς and and κράτος are in the genitive (as you probably noted), so the phrase could be rendered either as "the might of his power" or "the power of his might".

ἰσχύς is not very common in the New Testament, appearing only 10 times. It is related to the verb ἰσχύω which means to be able or have power. κράτος is just as common, appearing only 12 times. It is related to the verb κρατέω, which means to seize or arrest.

ἰσχύς is much more common in the Septuagint, where it appears over 300 times; whereas κράτος appears around 50 times. The two appear together in Deuteronomy 8:17, where Brenton proposes translations of "strength" and "power", respectively.

μὴ εἴπῃς ἐν τῇ καρδίᾳ σου Ἡ ἰσχύς μου καὶ τὸ κράτος τῆς χειρός μου ἐποίησέν μοι τὴν δύναμιν τὴν μεγάλην ταύτην

Lest thou shouldest say in thine heart, My strength, and the power of mine hand have wrought for me this great wealth.

So we might think of ἰσχύς as the quality or state of being strong and κράτος as an ability to act (following roughly COED definitions). But the two are so closely related - in English and in Greek - that you might be right in discerning a pleonasm.