[Sorry for the long wait, banged my head against a wall trying to get something to work on my computer. Failed. But ah well.]
Basic is that you stat 'em like you would a regular character (their progression being three hundred however) and then take the average amount of stats they would get during ten levels and then spread that evenly out during said ten levels (favoring the top if it happens to be unevenly arranged).
With the above you can tell that every ten levels they get six HP, four STR, zero MAG, six SKL, four DEF, two RES, three LCK, and five SPD. So as a rough rule of thumb, a ten percent progression means three total gains at level thirty, twenty percent is six, thirty is nine, so on and so forth. I don't suggest dealing with 5%s just to keep things simple.
Below is the rough guideline I used. Its not exact but its exact enough for this nature.
000%: 0; null
010%: 03; LVLs: 10, 20, 30
020%: 06; LVLs: 5, 10, 15, 20, 25, 30
030%: 09; LVLs: 3, 6, 10, 13, 16, 20, 23, 26, 30
040%: 12; LVLs: 2, 5, 7, 10, 12, 15, 17, 20, 22, 25, 27, 30
050%: 15; LVLs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30
060%: 18; LVLs: 2, 4, 5, 6, 9, 10, 12, 13, 15, 17, 18, 20, 21, 23, 25, 26, 28, 29
070%: 21; LVLs: 2, 4, 6, 7, 8, 10, 11, 12, 13, 15, 16, 17, 18, 20, 21, 22, 23, 25, 26, 27, 28
080%: 24; LVLs: 2, 3, 5, 6, 7, 8, 10, 11, 12, 13, 15, 16, 17, 18, 20, 21, 22, 23, 25, 26, 27, 28, 29, 30
090%: 27; LVLs: 2, 3, 5, 6, 7, 8, 9, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 22, 21, 22, 23, 24, 25, 26, 27, 28, 29
100%: 30; All levels
Now, with the above, we can stat this generic merc mook at any level. Why not ten? Fourteen? Four?
You can tool things around by increasing progressions, and I'd generally suggest making the enemies a few levels ahead of the average. I also haven't added the initial four bonus stats at character creation because I take that as a player only thing.