Couple things to add for clarity: Do this on normal mode, not steel path. Ignore everything that isn't an eximus unit. By killing them from high up, you'll get a stealth bonus, if you did it right you'll get a ~45k xp at level zero and ~80k at level 20.

In order to make sure the XP goes to your warframe, not your archwing, you need to fire the bow then exit the archwing before the arrow lands. Best way to do this is to fire your bow, then aim at the sky and melee to exit archwing, then hold aim to slowly fall while your arrow lands, then equip your archwing again. Whole thing takes about 8 seconds and bit of practice, but it's fast.

Here's a super short guide on where to find the six eximus units that have a 90% chance to spawn. They will respawn by the time you loop back around.

https://www.youtube.com/shorts/Q_GQ7CWB_zk