The top right says "dotsu" and the bottom left says "boaa." I don't think they are real words; like you said, it's onomatopoeia. It just means like "bam!" or "pow!" or something like that.

I've only been learning Japanese for a couple months. Since getting katakana down, it's been fascinating to see how different the onomatopoeia in manga in "translated." Even though they aren't real words, the translators in still feel the need to change them to something more recognizable to English readers. I'm not saying that's bad, just interesting to note. I was pretty fascinated when I came across one in Dr. Stone that was actually "translated" the same. I think it was "Waaaaaaaa!" or something.