only steel path endless is good source of affinity for less time spend, but to kill enemies there i highly doubt a new frame or weapon might be enough, unless you using a stealth farming method.
go for ESO public for ranking weapons , you will get some dps players and eventually rank your stuff up in 3-4 zones without boosters, do get smeeta kavat for extra luck.
for Warframes - i have 2 options, one is rinse and repeat of normal SO
second is, with helminth put baruuk's lull ability with its augment and do steel path exterminate.
lull will put enemies to sleep so go full max range, since your warframe would be unranked so ability's range would be less at start, so max range helps.
it's a stealth finisher, method so you rely on 500% stealth bonus affinity and with lull it's easy to maintain that. once you max your frame you can remove that lull ability, simple!