Try placing it in a drawer or cardboard box with little ventilation and let it overheat, repeating this a couple times should help heat the solder up until the motherboard /GPU connection warps and the system RROD's.
Although the timing is often strange, mine just RROD'd last weekend. I spent several months abusing it having it play Fable 2, Viva Pinata and watching movies constantly and it did fine, I let it sit 2 weeks play an hour of Armored Core Answer and it dies, luckily my 3 year RROD warranty was good through next month.