Terminator films states plasma weapons are extremely effective against Terminators. The T-X was designed to be an anti terminator terminator, and it's primary weapon was a plasma cannon.
In the second predator film they analyze the dart from the speargun, and conclude "it doesn’t correspond to anything on the periodic table and it has almost no weight but can cut through steel."
I imagine it'd cut through whatever terminators are made from as well, considering predators are a space faring species and thousands of years more advanced than anything in the terminator universe.