WIS saves and Perception are ok.
I was wondering about that. My reasoning for low WIS was, among other things, that it would limit Perception because it has only one eye and I liked the aggressive trait - sort of charge headlong into some commoners and engage in some slaughter without much ado or fear for retaliation because, well..., are you seriously going to come at me with those sharp pieces of mango? (Blackadder goes Forth, I believe)
So wouldn't a Perception proficiency sort of ruin that?
That's me not in favor of a Perception proficiency.
And I don't really know what a Wisdom proficiency is, so gonna have to check on that.
Even with Proficiency it's only a +1 skill.
Besides, being viciously aggressive hardly forbids a monster from noticing things well.
By Wisdom proficiency, I meant in Saving Throws. So it adds its proficiency bonus to its Wisdom saves. A creature's saves only use its ability scores in 5E unless it's specifically listed as improving them with its proficiency bonus. They don't automatically improve their saves as its Hit Dice increase like in earlier editions. Creatures with character classes will get Saving Throws improvements at certain levels (e.g. a Rogue starts with Dexterity & Intelligence and adds Wisdom at 15th-level when it gets
Slippery Mind and "you have acquired greater mental strength. You gain proficiency in Wisdom saving throws.").
Monsters usually don't have class levels in 5E, even NPCs tend to be treated as normal monsters (i.e. the human bandit Captain in the SRD is a humanoid with d8 HD because it's Medium sized, rather than a 10th-level fighter or whatever). Nor do they have to slavishly follow class level abilities, so an NPC "Master Burglar" might have
Saving Throws STR, DEX, WIS rather than the DEX, INT, WIS a high-level rogue gets. Oh, and they wouldn't have a "Slippery Mind" trait since there's no point, as the save proficiency would be listed in the stat block and the DM doesn't need to know where it came from (or have justification for it having one, for that matter).
My main reason for giving the cyclops a Wisdom save is a –1 is pretty piddly, so even with its advantage from a trait it wouldn't be very likely to save against a sight-based Illusion without proficiency added.
On thinking about it, it should have darkvision.
["Going-to-town" kicking in] No darkvision! [/"Going to town" kicking in]
Bummer!
In the immortal words of Battery Sergeant-Major Williams, "Oh Dear. How Sad. Never Mind."

