On the weekend my 10th mage used a rod of lesser empower metamagic with ray of enfeeblement to bring 2 fire giants to a Str of 16. I rolled a 10, x1.5 to get a 15.
This made it quite interesting for the DM to decide how heavy the half-plate armor and greatsword weighed, and whether the giants could stand under the weight when brought down to Str 16. In the end he decided that they collapsed under the weight of all the metal.
So I would say that Ray of Enfeeblement is definitely a good 1st level spell to bring down fighters.