Switching an inductive load, even with the flyback diode, is hard on the transistor, and you may be seeing an SOA (Safe Operating Area) issue that is causing the transistor to fail. When the transistor is turning off there will be a time during which the Vce is >30V and the current is still 1.2A. Here is the SOA for a beefier Darlington- the TIP122:
It's also possible that there is stray inductance in your setup (or inadequate bypassing of the 30V rail) that is causing the transistor to see overvoltage.
For the first problem, the most obvious solution is to use a beefier transistor (and, though I don't think it's the problem, you might want to use a 1N5819 rather than a 1N4007 for the flyback diode).
For the second problem, make sure your flyback diode is connected near to the transistor, and that there is sufficient capacitance on the +30V rail to prevent overvoltage. This is a function of the energy stored in the stray inductance. Alternatively, put a TVS directly across the transistor E-C leads.
You can use an oscilloscope and measure the E-C spike at turn off to determine if the second issue is a likely cause.