So today I read a book that was telling the difference between a daedra and aedra. It was in falkreath, hall of the dead.
One thing in the book that surprised me was that the aedra can be killed while the daedra can only be banished?
So now i am confused, how was akotash able to defeat mehrunes dagon then? Was he taking a big risk?

nah the aedra are already dead, but their still gods. Deadra are outsiders, when they die they are sent back to the waters of oblivion til their masters fetch them. Also in mundus the daedric lords don't have that much power, they seem to rely on mortal servants to do things.

also akatosh (or his avatar) beat dagon, there wasn't to much risk at all because aka was in his turf

The aedra are "dead" in the sense they scarified themselves to create Mundus. In reality, Nirn and Mundus are the bodies of the aedra. However, a fulled powered Alduin would eat everything and restart the cycle once again, even with the daedra.

With the princes, they're their own worlds, so everything on their realm is their rules and can do whatever they wish.

Lastly, the only real difference between mortals and daedra, is that daedra don't lose their memories upon death. Mortals, when they die, get soul scrubbed (save for certain factors, which will not be discussed) when they die. Best to think of a mortal's soul as like an etch-a-scetch. The doodling is life, and death would be shaking it.

It's a matter of place. On Nirn, an Aedra and Daedra fighting would go as seen in Oblivion. However, think of Shivering Isles. No matter how powerful you are, Sheogorath is stronger. Then Jyggalag is back and you are Sheogorath, and you are both equally Prince of that place so you can harm Jyg when no one else can, because in a Prince's plane of Oblivion, that Prince is God with a capital G and cannot be cast down.

It isn't really a matter of place. The two Et'ada of the highest subgradient are Aedra. Even without their immortality, they have many times the power of some Daedra. Some Aedra are considerably weaker than Aka And Lorkhan (if he counts as an Aedra), but we know that it was one of them — probably akatosh — who beat up Dagon. Since Akatosh MADE Dagon, its no surprise.

When Aedra die they go through the dreemsleeve and come right back, having lost their memory. They still come back though. Particularly prone to dying or being 'broken' are Akatosh and Arkay.