There is a square plastic bung In the bell housing pop it out and turn the engine over by hand (I use a big flat blade screw drive between then torque converter and gearbox) and flick it round, keep going until you see the drain plug put a tipex mark on the tc before you start so you know you have gone all the way round. Hope this makes sense.
It's a 5 or 6mm Allen key if its like my c230. If that's rounded you could try a torx or spline drive in it! But make sure you have to tap it in to get a good grip on it. Then id use an impact driver but not to hard as not to damage tc.
I take it you soaked with wd40. Could try mole grips on the outside really tight if there's enough access! Or left handed tap and bolt or a centre punch to see if you can move it. Sounds like its been overtightened at some point. Drilling is really last option as swarf could enter and that's not good. Maybe a wide flathead bit might force in with an impact driver. Good luck.