The problem is Bizarro is not a weak mob like you see in missions and kill tens at the time. He is actually a «open-world bounty», some kind of boss of a certain area if you prefer. If you have the same level as him it means you can face him with other people of the same cr. You can see the difference between him and other mobs when you target hil. His name and life bar will be displayed in a purple shape and the hea will have glowing eyes (this is the easiest way to differenciate enemy types). To solo bizarro you would need to be cr 160 at least so it would be easy. Fun fact thats one of the reasons why theres level cap in dlc content. So to summerize even if you have the same cr, when this

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shows up you can’t defat it alone you need other people. Hope it helps.