A surface channel waveguide (WG) laser is produced by diamond saw dicing of a 15 μm thick 10 at. % Tm:KY1-x-yGdxLuy(WO4)2 monoclinic double tungstate thin film grown by liquid phase epitaxy on an undoped KY(WO4)2 substrate. The WG propagation losses are 1.1±0.5 dB/cm. When pumped at 802 nm, laser operation is achieved with a maximum output power of 262 mW at 1833 nm with a record slope efficiency of 82.6% (versus the absorbed pump power) in a TE10 spatial mode (linear laser polarization, E‖Nm). Diamond saw dicing of double tungstate epitaxies is a promising technology for manufacturing WGs for sensing applications.