Some of them occur too early IMO, but they can still be done. On Darkstar, the ridiculous 4 phaetons on round 2 proved to be a challenge a bunch of randoms and I only beat with 1 second left. Part of that was lack of coordination, though. It everyone would've been in warthogs, we would've easily taken them down.
As for the 3 wardens, I played w/ a random team one game on Apex. I had a Corp Scorpion, someone in a regular scorpion, and one in a wraith. Took maybe, MAYBE 3, 3.5 minutes to beat them. Nothing too bad.
Just find people to play with who are willing to use vehicles instead of saving their "precious REQs"