The problem with this is that she isn't really niche. Her cost is barely higher than mages and those you still mange to place during maps, right? So if you can skip placing Gellius + a mage and just place her, then provided she can tank whatever comes her way, she will be worth 2-3 mages in terms of dps AND will not need to block because she will brutally murder things before they get a chance to go past her, with her high attack speed.
Edit:
In either case none of the two are placed down on a map asap, really.