Yes, Baldur will benefit from being paired with a minion summoner. I struggle to choose between Santa and Noor in a vacuum.
Santa is tankier and has the damage, attack down and defense down while Noorâs secret sauce is her anti-minion minions. After Santa fires Baldur hits for much more damage and you take less damage.
Noor has beefiest minions with a good attack stat.
If that were the end of the discussion I would be inclined to tip Santa.
You mentioned the Christmas event, there Noor has the advantage because of the legions of minions the bosses can spam you with in a protracted battle.
If you arrive at the boss stage with Santa charged, apply the debuffs along with a wall of minions on your team from the mob stages, you should sail through smoothly.
You also mentioned you have Freya. She adds significantly to the value of minions as you are aware and Noorâs minions have the attack stat to really benefit as well as being tough enough to stick around to do damage.
With C-Rigard boosting Baldurâs attack he should do reasonable damage. Freyaâs boost to the minions and you have a solid DoT coming from your team.
I believe that Santa will help end the battle more quickly than Noor after firing their specials, Noor does a bit more to keep you alive until that point.
Really donât think you need to use Grimble with Noor, her minions eliminate minions leaving Grimble doing minor damage and hardly any mana boost, she cuts him off at the knees. He would pair decently with Santa.
Your 5th hero slot is flexible. A damage dealer (sniper or hit three) brings good synergies. A debuffer is always nice, if looking at the Christmas event (also Halloween) a second cleanser would be handy. You have a stall team so a mana controller would also have good synergies (Proteus has the poison which blends nicely with you Baldur/minion DoTs).
DEFENSE: Santa is a legit tank and can cause real problems. Baldur and Noor have offered zero resistance when defending (have seen very few). If you plan to use Santa or Noor to defend the Stronghold it should be Santa.
Full disclosure:
Santa, Noor and Baldur are all sitting at 3-70 in my roster, Santa for over two years. I have tested them on maps and in some raids. None of those venues played to the strengths of Noor and Baldur so I have not seen them shine for me (partly my team composition was not optimal and partly b/c facing teams that are mostly +20 is not a good test of heroes at 3-70).