にせよ, and にしろ are two phrases that are often translated as ‘even if (A)’, or ‘no matter if (A)’. This structure is a combination of に, and either of the imperative forms of する; せよ, or しろ. せよ tends to be more common in written material, while しろ is more common in speech. Despite the English translations, this particular use of the imperative form of する translates more accurately to ‘so it’s (A), who cares!’, as the speaker is just strongly stating that the actual condition of (A) is unimportant in relation to what they actually want to say.
にせよ and にしろ can appear following verbs, い-Adjectives, な-Adjectives or nouns in any of their standard forms.

• 飲（の）み会（かい）に参加（さんか）しないにせよ、参加費（さんかひ）は明日（あした）までに持（も）ってきてください。
  Even if you are not going to attend the drinking party, please bring your entrance fee by tomorrow.
• 怖（こわ）いにしろ、私（わたし）の仕事（しごと）なのでやらなくてはならない。
  Even though it scares me, I have to do it because it's my job.
• どんなに不便（ふべん）であるにせよ、私（わたし）は自然（しぜん）が豊（ゆた）かな田舎（いなか）に住（す）みたいです。
  Even if it would be very inconvenient, I would prefer to live in the countryside where nature is abundant.
• あの人（ひと）は相手（あいて）が子供（こども）にしろ、一切（いっさい）手加減（てかげん）はしない。
  That person does not take it easy at all, even if the opponent is a child.

As this structure is fairly direct, in most cases it will be used as a way to emphasize that regardless of what the case is for (A), (B) is still true, or is the more important matter to consider.