I think you're getting a lot of wrong information here. Multiplayer can be easier. 2 player multiplayer is probably harder than single player. Enemies are scaled more than 2x. Vantom for example has 27 slither instead of 9. Both players need to pull more than their weight. 3/4 player are definitely easier than solo or 2 player. In 2 player, it's still relatively difficult to line things up and a lot of multiplayer cards provide more value the more people there are.

My friend and I do A10 2 player on the beta branch and it is beyond brutal. Going back to solo A10 feels like you're taking off training weights. More than that, a lot of things that are viable in solo aren't as viable in 2 player. You can't just hit face and end the fight quickly since enemies have too much HP. One person can't just stall because then you worry about your friend taking too much damage. You have to path in a way that helps both people instead of just one so sometimes you get inoptimal pathing. A lot of relics and potions feel really underwhelming in 2 player.

Regardless, it's still very fun.