Kill enemies to increase time.
Since you need to defeat enemies quickly to extend the timer, concentrate on taking out the grunts first before concentrating on a tougher target. Take note that the grunt enemies per wave isn’t unlimited so after killing them, you’ll have to defeat the “wave boss” with the remaining time.
In the first wave, you’ll deal with reavers and a traveler. Take out the reavers and use your runic attacks against the traveler whenever he’s open or finishing up an attack. Do not do this when he’s in a defensive stance as his shield will completely mitigate any of your attacks.
For the second wave, you’ll have to deal with nightmares and wulvers. Wulvers can easily avoid runic attacks so the best way to deal with them is by parrying or blocking their attacks then counterattacking with runic attacks. Atreus can pretty much melt the nightmares
For the last wave, you’ll have to deal with draugrs and a soul devourer. Ranged runic attacks are best used here since they can critically damage the devourer if they land on its weak spot.
Avoid taking damage
Getting even a tiny bit of damage (especially when you’re doing a runic attack) will fail the challenge. For good measure, set ranged runic attacks, use axe throws, and let Atreus go crazy with shooting enemies from a distance. Always keep an eye out of your threat indicators to avoid getting sniped by ranged enemies. As long as you maintain your distance and avoid melee engagements, you should be able to win this challenge.