If I'm not mistaken, にせよ is more or less にしろ and they can be used like にしても.
The thing I don't understand is that normally in Xにする, X is a noun but in this sentence にせよ is used right after a verb in dictionary form(without よう or anything like that) :
何をやるにせよ、ベストを尽くしなさい。 No matter what you do, do your best.
