It's certainly some thing to test more, people need to have more matches where they specifically try different matchups, instead of playing them all normally.
But of course 10 Grenadiers will beat 1 Devastator. Look at the actual numbers (thanks, spreadsheet!).
10 Grenadiers is 50 Recruits/50 Support, a Devastator is worth 18/15 support - less than half that. If someone invests more than twice the amount of recruits and three times the support, it's not a surprise that they can win.
It will take 1 Devastator 4 normal shots to kill a Grenadier, but if you add a second Devastator (36 Rec/30 support) versus those 10 Grenadiers, then it'll take 2 shots from the pair to kill a grenadier, and with the splash damage it can hit more at once.
It will take one Grenadier 25 shots to kill a Devastator, and your group of 10 will need to shoot 2.5 times.
Tanks (Armours) are explicitly listed in the manual as a counter to Devastators, it's intended that they be strong against them.
The lesson here is that you need more than one Devastator.
