Companions from base game have abilities "_q403Follower" and "MinDamage". If I remember correctly, in OG their attack power is reduced down to 20%. There are 2 ways to modify this:

1) Edit configs (correct way):
Extract contents of "modNoLevels\content\blob0.bundle" file. In file "gameplay\abilities\opp_base_abl.xml" edit ability "_q403Follower". Change attack_power from 0.5 (+50%) to 0.0 or -0.8 (-80%). Do the same for "MinDamage". There is also a copy of this config in "abilities_plus" folder for "New Game +" mode. Pack everything back.

2) Insert hardcode into scripts (easy way):
Open script "modNoLevels\content\scripts\game\gameplay\damage\damageManagerProcessor.ws", find code:
powerMod = GetAttackersPowerMod();
replace it with
powerMod = GetAttackersPowerMod();
if (!playerAttacker && (actorAttacker.HasAbility('_q403Follower') || actorAttacker.HasAbility('MinDamage')))
    powerMod.valueMultiplicative = 0.2;

npcVsNpcMult in NL v1.4.3+ works differently, it affects everyone except Geralt and Ciri.