knowing how a unit works in theory and how she works in actual usage are 2 different things. just sayin'
I know the former, I have no idea about the latter (since I don't memorize dorothea's normal and AW skill; not to mention control over her switching skill requires veteran ability to use it well). regardless, blacks are blacks and will pretty much trump over most other options a free player has. tentatively I have classified her under assist dps/heal unit, in any case, since i'm not sure how reliable her heal/dps is in actual practice.
as a free player, I don't have access to many units and I don't blindly grind challenges (majin advents) to improve myself due to resource constraints. True veterans are so used to this game they are able to tell when a skill is about to end as well as manage unit skill cycles stacking and overlapping as needed.