Dragon Tail has -6 priority, so Quagsire will always hit first with its moves (all of which have 0 priority). That said, if you're still stuck at Terra, then you could always train a new Pokemon to change the terrain.
Depending on what you have in your box, any Pokemon that learns either Electric Terrain / Mist / Misty Terrain will do the trick. Since you have no Heart Scales, the only options would be Manectric and Vanilluxe.
Alternatively, you could just mine the rocks at Ametrine / Celestine mountain and try to get a Heart Scale from those (soft-resetting should do i
