best mission to farm Challenging Mode is the Lexington Event Center which you can finish in ~15 minutes with the correct skills picked through your team, means 2 Phoenix Credits a minute, even more if its a daily mission
good teamplay and you can clear it even faster, a bad run should not take more than 20 minutes
go all out on damage, healing skills will just mean the mission take longer +
healing primarily is done via medikits anyhow since they are much faster
1 scanner skill to indentify the critial opponents that need to be stunned
2 stun grenade skills to stun the new waves directly on spawn
4 damage boosts so new waves can be killed right out of their spawn while stunned
4 heal skills that allow you to revive teammates from the distance
another tipp is to always go for the backpacks on the big ones which greatly increases the time to get the down, just going for headshots takes way longer
here a short video in which you can see in action what i just explained
once you figured out the spawn-waves and locations its quite simple
https://www.youtube.com/watch?v=TpAwgL9x8U0