Heat vs derating power resistor Lithium battery discharge load tester
An ideal load for discharging a battery at a constant level is a constant current sink (or constant current load). Usually built with an opamp and a transistor (which gets hot) and a current sense resistor and a voltage reference.

Resistors can be destroyed even when operating below the rated wattage if the cooling is not good enough. The copper connection to the resistor plays a big role in cooling. So a fat trace or even a plane can help to cool the resistor down.

In a datasheet you'll find typical values to calculate the temperature of your resistor based on certain copper areas connected to the pads. (actually quite hard to find)

Most resistors are able to handle their rated wattage at an ambient temperature of 70 °C, so they are designed to get burning hot. Vishay power resistors for example are able to handle 170 °C.

FR4 is able to handle 115-140 °C, so I'd still try to spread the heat with some copper areas around the resistor pads (connected to the pad) to reduce the temperature of the FR4. If you can, areas on both sides connected with vias will help even more to spread the heat and increase the cooling effect.