No, I was connected to steam most of the time. i.e if i for example slew Torax 3 times i am absolutely sure I was connected during 2/3 times. 

Yes I did slay them within their territory. My issue seems to be very similar to these ones below 

From Fallen Enchantress Forum (Sorry did not find this earlier) :  https://forums.elementalgame.com/446386/page/1/  and look for "[Bug] Abeix/Delin/Torax/Vetrar Steam Achievements"

From Steam Community Forum: https://steamcommunity.com/app/228260/discussions/0/558751812771182476/

 I guess there are others with the same problem but i cannot find a solution online.. I completed all other achievements except for these bosses. I know the steam achievements don't mean much, but still for some annoying reason I wanna get them all.

Perhaps this is the issue here, your save file.

When I get an achievement legitimately, but it doesn't trigger for steam, if I care enough there's usually save files located somewhere that are dedicated to steam achievement hunters for this sort of purpose.

Alternatively, launching it with cheats enabled may or may not block achievements; if it doesn't, it shouldn't be too hard to get it that way. If it does block achievements? I don't know what to tell you.

The last thing is that I've found repeatedly saving/loading FE:LH is pretty unstable - playing it on a psuedo-ironman mode usually leads to better results (and a harder game  )

Ironman modes for games generally are when saving/loading is restricted to exiting the game, or not being able to save at all. Basically, if something goes wrong, too bad, deal with it, and zero reloading for any reason whatsoever.

Champion glitched and disappeared? Sucks to be you (but it happens to the AI too occasionally). Misclicked in diplomacy and declared war? Well, I hope you're on war footing. Got really unlucky in a battle and lost? No retrying

There's no actual setting for it in the game, but that's why it's "psuedo-ironman"; you just have to keep yourself from reloading an earlier state of the game, which also tends to help with stability (or so I've found).