kspes: good, looks like you're getting close!
so, the achievement should unlock after you kill EACH enemy unit type from EACH race.
The "within one match" restriction has been lifted, you can achieve this through any number of matches.
because this restriction has been lifted, we decided to balance it out to include all races ;)
each of the races has 9 unity types, according to the contents of your achInfo.cfg file, you've taken care of all english types, 4/9 french types and 5/9 viking types.
If you want to cheat, you can add all but one type of these from my first post (make sure to prefix them with E_, V_, F_, corresponding to the respective race), to have a total of 26 (suggest to leave out the clubman), then kill that one and you'll get to unlock the achievement.
or, of course, keep doing it the hard way, it's more fun and rewarding :)
Edit: I can confirm it doesn't work Human vs Human. I guess it has to be vs CPU. It took me 3 days on one map to kill all the English units while waiting for monk and catapult units to spawn, i guess i will have to do the same for French and Vikings. >.<