Bruticus and Guardian (Defensor) had the following encounters in Japanese G1:

* FSRLT※ manga #5 (1986): Defensor fights Bruticus in a Japanese baseball stadium

* Scramble City OVA: Defensor appears and stares down Bruticus and... uh... not much else. They don't actually fight. Bruticus and Menasor are defeated by Metroplex. Defensor just points at Bruticus. Robot points.

* The Story of FSRLT※ manga, issue #5: Defensor, Superion and Omega Supreme battle Bruticus, Menasor and Devastator. The Autobots are all defeated by Predaking. Predaking is eventually defeated by the Omnibots. Robot Points!

* The Headmasters episode 3 (Birth of the Fantastic Double Optimus Prime): Defensor fights Bruticus in the distant future year of 2011.

* The Headmasters episodes 9 & 10 (Cybertron in Imminent Danger, Parts 1 & 2): Defensor fights Bruticus on Cybertron. While this fight happened over two episodes it was the one fight (so it counts as one if we're counting encounters).

* The Headmasters episode 34 (The Final Showdown on Earth, Part 1): Defensor fights Bruticus in Canberra, ACT. Defensor is defeated.

That's 6 times that Defensor encountered Bruticus in Japanese G1, 5 of which they actually fought. While he did encounter and fight Bruticus the most, he wasn't his sole nemesis as he also did fight Devastator, Menasor and Predaking.

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※Fight! Super Robot Lifeforms Transformers (戦え！超ロボット生命体トランスフォーマー)
P.S.: Yes, yes, I am aware that the Omnibots (Double Changers) weren't mail away exclusives in Japan. It was a joke.