Appreciate the comment @riandeuk. I've often used the Earth's 'Failed Summoner' in my fair share of battles against magic attacks, but it hardly made a difference against '+1' water summoner.
Maybe I was not too impressed by a monster with this ability to really try Owster Rotwell enough, but you know what, I'll give it a go, and won't hesitate to add it in here (crediting you of course) if it handles these attacks more efficiently.
Looking forward to trying it in the next few battles! I'll keep you posted in this thread as to how it goes.