I know you need to encounter non epic monsters that are on the same space as an epic one, but do you have to defeat each non epic monster to fight the epic monster?

Quick answer; yes.

Now I go dig up the rules... be back in a sec.

During the Encounter Phase, an investigator must encounter each non-Epic Monster on his space before encountering each Epic Monster on his space.

That shows up in the "Additional Rules" section of every expansion.

Supposedly since it doesn't show up in the errata if you are just playing with the base game then you can choose to face the Epic Monster first... But that wouldn't be my interpretation.

I know you need to encounter non epic monsters that are on the same space as an epic one, but do you have to defeat each non epic monster to fight the epic monster?

During the Encounter Phase, an investigator must encounter each non-Epic Monster on his space before encountering each Epic Monster on his space.

That shows up in the "Additional Rules" section of every expansion.

Supposedly since it doesn't show up in the errata if you are just playing with the base game then you can choose to face the Epic Monster first... But that wouldn't be my interpretation.

One minor, but important clarification: as stated you only need to encounter the non epic monsters first. So, to answer you original question, you don't need to defeat the non epic monsters to fight the epic monster - just encounter them. Of course, if you don't defeat the epic monster, then next turn you'll need to encounter any non epic monsters you didn't defeat all the prior turn all over again before you encounter the epic monster again.

I know you need to encounter non epic monsters that are on the same space as an epic one, but do you have to defeat each non epic monster to fight the epic monster?

During the Encounter Phase, an investigator must encounter each non-Epic Monster on his space before encountering each Epic Monster on his space.

That shows up in the "Additional Rules" section of every expansion.

Supposedly since it doesn't show up in the errata if you are just playing with the base game then you can choose to face the Epic Monster first... But that wouldn't be my interpretation.

One minor, but important clarification: as stated you only need to encounter the non epic monsters first. So, to answer you original question, you don't need to defeat the non epic monsters to fight the epic monster - just encounter them. Of course, if you don't defeat the epic monster, then next turn you'll need to encounter any non epic monsters you didn't defeat all the prior turn all over again before you encounter the epic monster again.

Okay that makes sense then.

Okay which begs the next question do you only have to encounter a monster to have a follow up location encounter, or do you need to defeat the monster?

Okay which begs the next question do you only have to encounter a monster to have a follow up location encounter, or do you need to defeat the monster?

You must have defeated all monsters at a location before you can have any other encounters there. Reference Guide page 3, Combat Encounters (last entry):

If an investigator defeats every Monster on his space during the Encounter Phase, he may resolve an additional encounter of his choice.