actually a plasma cutter is about 50,000 degrees F if i remember right, and that is almost 17 times hotter than it takes to melt steel. and the reason i brought up a lightning strike, is that if a plasma cutter can do what you are saying it can, then a lightning strike would most definitely melt a car instantly because it is so much hotter than a plasma cutter. yes the plasma cutter cuts through anything pretty much instantly, but if the object is large enough like...idk, rex's leg, it would take quite some time to get through it, one of its lets is like the thickness of a tank, and is not hollow, it is the support for that 300+ ton monster.
Edit: not cut through ANYTHING just any type of metal