You could solo most missions, there's a doll NM in Delkfutt you won't be able to solo, Detector NMs in Misreaux, And a Bugard NM, Tenzen and anything from 8-1+ :x But you could duo those!
You can easily solo up to Id say personally 5-3 on any well geared 75 I did on WHM.
As for that onward. I would say easily duoed I trioed Tenzen with two other well geared players. I wouldn't risk it if you are halfassed but it can be done.