If you multiclass monk thief-rogue and 1 level of something to give you heavy armor, you end up with 20+ AC that deals 100+ damage per turn at level 9.. Also dual wielding fighter + thief-rogue can do a lot of damage especially on first round of combat.
Which brings me to my point of this post, there is a lot of spells and abilities that give you buffs for multiple rounds.. i tend to finish 90% of opponents in 1-3 rounds on tactician mode making them quite pointless to use an action on. Just like a lot of spells are quite pointless to spend entire action on them, they are so weak that bonus action would be more appropriate and it would also buff the caster classes a bit so they can cast 1 useful spell and 1 spell that is crap but is fun variety spice so you dont always use just 1-2 spells every combat.
Most difficult battles is where someone mind controls my fightermonkrogue combo and then he/she kills one of my party members on first round, then i end up with just 2 playable characters as one is dead and one is mindcontrolled
