These things are always a pain.
There is no way the starter can do the job.
I have had no luck using impact wrenches for this purpose either.
As MBDoc said, a flywheel lock is probably the right tool here, though I have never owned one. I presume you remove the starter to gain access to the flywheel ring gear, which is not always a desirable step, depending...

1) Is the pan off? You can usually put a block of wood between a connecting rod and the block to stop the motor from turning. This has been the most successful method for me, even when everything else failed.

2) A big strap wrench around the pulley worked once. It was designed just for this. Can be hard to find. I got mine at Harbor Freight. Has a V-belt, but works ok on serp belt pulleys too.

3) Before I had a strap wrench (which is gentler), or when the strap wrench is just not enough, I sometimes have success using a big chain-wrench for this. Now this thing gives you some serious counter-torque. It can damage the pulley surface, though it helps to put a rag or an old segment of belt under it. I got the chain wrench at Sears. I was able to wedge the chain wrench against the floor to stop the pulley from turning, then put a big pipe on the breaker bar, and presto...

4) In a pinch, I once used an old piece of serp belt and a pair of big visegrips. Wrapped the serp belt around the pulley, clamped on the vise grips real tight, and was able to stop the pulley from turning by tying the tails of the belt to something solid. That particular bolt was only torqued to 90lbs, though, so I doubt it would work here. Belt might break or slip first!

Good luck. I feel your pain...

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1986 300E 5-Speed 240k mi.