Both are most easily done while playing solo. The challenges do require some luck though. The scavenging can be completed after the 2nd barrier hive is down since aliens never spawn after any of the big 3.
That's actually easy man. I notice in all my time playing. Not taking any seeker dmg or kill x amount in the air is my nightmare. Also helps if people had the turrets. Best thing ever on there. I'm going to be on after 1am. So I'll try again lol.
Leaper isn't difficult. Just scan for the fat alien that's trying to flee.
To live or die!!!! Thats just my style. psn- kozaka