I must be really unlucky then
After round 5 i never get it again. My thoughts on the damage issue are:
a) I think should be accorded to overall commander stats. A commander with 8000 HP can not get +8 to damage if damage card is offered every 100 battle turns.
b) I think it should be flagged. it should be proposed one time at least every 50 turns. So you get it 4 times for sure if battle = 200 turns.
Well, is your mod, so I can do tiny modifications only for my game play. For now, I modified the damage to be commander HP:100. So, at start (200%) my commander gets 15 damage, as he starts with 1500 HP. If he receives this card at 8000 HP it should be +80 to damage.
But the card is never offered again, or only once, so I have him often at 8000 HP with only 44 damage, which does not seem to me very realistic. He barely can kill 30 pixies.
With only 44 damage but 170 HP/turn regeneration, there are two issues:
a) he cannot die because the monsters generated can not deal more damage/round
b) he cannot win because he deals too little damage and AI keeps summoning.
Of course, you can go the safe way: get necro, amass 800 zombies-keep them under antimagic-, then destroy the other commander in 4-5 turns. But is a bit boring to use same tactic.