A bit late I know, but I got this last night. I thought I'd levelled past all the throwable enemies until I decided to try betraying Roah Moonglow and her gnome bodyguard. I thought you had to throw one enemy at the other, probably because I've occasionally thrown objects but never used improvised weapons. Throwing doesn't give an achievement. But as soon as Shadowheart picked up Phase and slapped Roah with him (as an improvised weapon), the achievement came up. Both had plenty of health left.
I use a handful of mods, mostly to simulate a first-person view - nothing to alter difficulty; and I have the re-enable achievements mod too. I use the GOG version.