Barbarian magic suppression skills...

For this reason, if the choice is "either/or" (although you can, of course, max out both), I believe it is more advisable to invest in Anti-Darkness and not invest in Anti-Summoning!
That's my overall thought. Are you going to argue with that?

Of course;)

Look: In the replay, my Special Points (SP) are around 44, right? In the battle I described, Leto's SP was 51. Without any agreements – the map is like that. Accordingly, the Phoenix's parameters were even higher. Out of all the Darkness spells, I only used Subdue on the Wolf Riders and Vampirism on the Phoenix in the battle. How would Anti-Darkness weaken these spells? Practically not at all. But Anti-Summoning would severely reduce the Phoenix's parameters and decrease its Shield. In addition to the Shield having fewer hit points, the absorption would be 25% instead of 50% – a huge difference, wouldn't you agree?
And I think investing in 2 Slow spells right away is pointless. It won't be as effective, and you'll have to give up something else...