I can certainly see why this is considered brutal but there's certainly a few things I'd change.
Firstly I'd take a few vets as troops choices, the BS4 really helps in comparison to the regular troops and you have the option to add three special wepons if you wish, I agree with the commissars as an addition I find them really useful, I would take less
LRBTs and bring along either some manticores or some basilisks.
I strongly recommend bringing some storm troopers either in one of the vendettas or as a deepstriking unit to distract the opposition, this has worked really well for me a few times.
The vendettas are always a good choice and maybe switch one autocannon out in each platoon for a lascannon for those tough enemies.
Another thing you could do with the infantry platoons is blob them together and give some vet sergeants some power weapons for tarpitting monstrous creatures.
Hope this helps