Did the workout below then Tim showed up and we worked on his technique for an hour or more. Primarily focused on hand entry and preventing the hands from crossing the center line during extension.

Warm up
3 sets as:
2x100 free
4x50 stroke or kick
all with 10s recovery

4x100 free as:
25 drill/75 swim
15s recovery

Main set at 85% effort as:
1x400
1x300
1x200
4x50
10-30s recovery

Cool Down
4x75 easy pull
6x50 choice