With the kind of dps she pulls out i'd argue most things in existence will be deathinated before they manage to run past her. Don't forget she has a much faster attack speed than mages while dealing a lot more damage per hit as well.

That said, Gellius will need an UP battery too, most units tend to do, it IS the first thing you place after all. But depending on the map you will either need to place a lot of units fast or have a decent amount of time before the next big wave comes in. That is basically the difference between deploying Nadia and something else.