にしても

にしても means "even if" or "granted that": you accept a point and say that it changes nothing. It goes after the plain form of a verb or an い-adjective, and straight after a noun or a な-adjective.

The verb or adjective stays in the plain form, not the て-form that てもtakes. A noun or a な-adjective takes no だ.

• × 忙しくてにしても → 忙しいにしても
• × 冗談だにしても → 冗談にしても

Even so, this is too much

にしても often grants the circumstances and then complains that the result still goes too far. The second half carries the criticism, often with ぐらい or すぎる.

Whether A or B

Said twice, A にしても B にしても means "whether A or B".

Register

にしても is at home in speech and in writing. にしろ and にせよmean the same and are stiffer, にせよ most of all. としても is closer to a pure supposition, "even supposing that".