Monsters use the standard move, action, Bonus Action, and Reaction, as in 5e. They do NOT use Effort or Dynamic Reactions. A DM can use any 5e compatible monster book or stat block as written. Only two small tweaks are made and can be done easily, on the fly:

Monster Attacks

Like PCs, monsters simply roll their damage dice + stat modifier. They miss on an attack by rolling a 1 on their Primary Die, and crit by rolling the maximum. However, they do NOT use saving attacks.

Monster Armor Class

Unlike PCs, monster armor is simplified into three groups:

• (0-13 AC) Light armor. Takes damage as expected in 5e.
• (14-17 AC) Medium armor. Takes damage ONLY from the dice, ignoring ALL damage modifiers (e.g., STR/DEX bonuses) unless they are negative.
• (18+ AC) Heavy armor. Takes half damage from all sources (rounded up).

PC Critical Hits, saving throw spells, and damage type vulnerabilities ignore monster armor altogether.

Minions

Use a single damage die for each minion and roll them all at the same time: 1d4 for weak minions, up to 1d12 for a stronger minion. Minions do not use modifiers, they miss on a 1 and do not crit. PCs can Defend against multiple minion attacks as if they were a single attack. ANY damage kills minions, and a Critical Hit can kill more than one if it makes sense.