How do I survive more than 30 mins?

Newer player looking for advice. I see these challenges to unlock things that say survive 31 mins on “whatever” map. How would I be able to kill the reaper in order to do so? I’m assuming there are builds strong enough, can someone help me know which builds are best? I doubt I have everything I need unlocked but at least I’ll know what to grind for. Any tips are extremely appreciated.