Is there any "standard" method (by which I mean without using a special action "card") for NPCs do do critical damage besides the (rather unlikely) event of rolling a comet?

i am not sure i understand your question but...

how to crit w/ monsters

• if they are using swords then i assume the swords regular damage and crit rating applies
• they have monster actions like the rat ogre that tell you when they crit like "bone 'rippin"
• you can give them standard action cards up to their expertise plus one which will tell you when they crit
• you can use their expertise pool to assign expertise dice when appropriate, not real likely to crit just doing this

Well, what struck me was that the stat blocks for monsters have damage ratings but not crit ratings. So it didn't seem like they could do critical damage with a standard attack by getting enough boons. That's what I was asking about.

CR3 for 'typical' attacks isn't out of the realm of possibility. 3's about 'swordish' chance for a crit, and to me average, so think about using some standard crit rating for NPC/monster attacks. Obviously if you're equipping them with extra gear or they state they come with a weapon of some sort use that actual CR instead (say equipping a cultist with a bow, or a skaven with throwing weapons).

I've just got to equipping my monsters. I wasn't criting much, then I put the weapons CR rating in my notes. Bam, it's a new ball game.