二人を(ふたりを)騙して(だまして)ひどい目に(ひどいめに)遭わせよう(あわせよう)と企んでいた(たくらんでいた)

Also living in that forest was a cunning 'raccoon dog' (Tanuki). Good at tricking other animals, the raccoon dog plotted, 'Compared to my wisdom, both the spider and the slug are just tiny bugs. Someday I'll trick them and give them a terrible time.' The three looked down on each other and acted arrogant.