Current limiting at mains voltage
Use a high power resistor. For any given temperature, resistors will be more reliable than semiconductors, so there is no advantage in dissipating that power in a transistor.

This is common in power circuit design, for example dimmers and switched mode power supplies, you will need high power rated components because you are working at high voltages so even moderate currents produce high power dissipation. 

I assume you are using a hard-fired technique, where you leave the gate current on continuously. In that case, the only way to reduce dissipation in the triac gate resistor is to use a more sensitive triac. But there is also the soft fired technique, where you only pulse the gate at the zero crossing and the triac stays latched on until the main current falls to zero. This is not suitable for all loads.