Decided to play around with the Alien Archive monster creation rules by converting all the Pathfinder Bestiary 1 demons and devils to Starfinder. Because they're my favorite monsters and Alien Archive is decidedly lacking in both. This is my first attempt, and I tried to stick to the original monster stats and follow the monster creation rules as closely as possible.
The only thing that really caused a problem was what to do about the original dretch's stinking cloud. The spell doesn't exist in Starfinder, and in Pathfinder it's a 3rd-level spell. I decided to give it an ability which mimicked the closest thing I could find, the smoke grenade in the Core Rulebook. Maybe I could have done better?
Also the Alien Archive doesn't mention how to assign monsters Damage Reduction. It seems like it should have been in the Universal Monster Rules but was skipped over for some reason. It didn't seem like a stretch to give this monster the same DR it had in Pathfinder as one of its two special abilities, so that's just what I did.
DRETCH -- CR 2
XP 600
CE Small outsider (chaotic, demon, evil, extraplanar)
Init +0; Senses darkvision 60 ft.; Perception +7
DEFENSE -- HP 25
EAC 13, KAC 15
Fort +6, Ref +4, Will +1
DR 5/cold iron or good; Immunities electricity, poison; Resistances acid 10, cold 10, fire 10
OFFENSE
Speed 30 ft.
Melee Claw +11 (1d6+4)
Offensive Abilities stinking cloud
Spell-Like Abilities (CL 2)
-- 1/day — fear (DC 11), summon allies (1 dretch 35%)
STATISTICS
Str +2; Dex +0; Con +4; Int -3; Wis +1; Cha +1
Skills Escape Artist +7, Perception +7, Stealth +12
Languages Abyssal (cannot speak); telepathy 100 ft. (limited to Abyssal-speaking targets)
ECOLOGY
Environment any (Abyss)
Organization solitary, pair, gange (3-5), crowd (6-12), or mob (13+)
SPECIAL ABILITIES
Stinking Cloud (Su) Once per day a dretch can release stinking gas from various orifices in its body, creating a cloud of dense, yellow-green fog. The size and effects of the cloud are identical to a smoke grenade (Starfinder Core Rulebook 184).
I have a couple of comments about how your monster is different from what I see in the NPC creation guidelines. Varying from the guidelines isn't a bad thing, but it doesn't hurt to ponder the differences.
- DR indeed doesn't have any guidelines. A pity. Some creatures get DR 5/magic as low as CR 1/2 or 1 (undead minion). DR 5/adamantine doesn't show up until CR 9 (scavenger slime) and DR 5/- or DR 5/alignment doesn't show up until CR 5 (elementals, summoned creatures). And I don't think I've seen cold iron weapons in a Starfinder product.
- Your critter has an extra +1 ability score bonus (in either Wis or Cha).
- Perception is automatically a good skill, so you can get another good skill, if you want or need it. (And, picky detal, Perception isn't supposed to be listed on the Skills line.)
- The smoke grenade is like a short-range, short-duration fog cloud, a 2nd-level spell, with an extra rider effect.
- It might be a good idea to specify that fear is the 1st-level version of the spell.
It think it's about right, overall. Maybe the DR is a bit much. Between DR 5, 1 energy immunity and 3 energy resistance 10, what low-level weapons wouldn't have their damage reduced? [Are holy weapons common for PCs of about that level?]
Cold Iron is one of the listed weapon materials on P. 191 of the CRB. +450 credits for a weapon, +9 per cartridge for projectiles. Holy is a level 2 fusion, so it's available if a PC is willing to grab a level 2 weapon to hold it. DR 5 is fine, so long as you're careful about it.
The 'extra' ability point is fine even without considering GM fiat and deviation, since the only guideline for the second three ability modifiers is essentially "don't make these bigger than the first three." It is a little better off than its Pathfinder counterpart, though.
I would actually be slightly wary of a smoke grenade as Stinking Cloud. The smoke grenade is potentially stronger than porting the spell directly; nausea is crippling, but the grenade bars any action on a failed save. The DC will also be higher - converting Stinking Cloud as a 3rd level spell would be DC 13 or 14, while the grenade is DC 15 +1 per previous check. Perhaps that's countered by the prevalence of environmental protections, though.
Note that having it as Su means that it no longer provokes an attack of opportunity, unlike the Pathfinder Dretch's Sp.
Beyond that, as Distant Scholar notes, you're free to give it one more Good skill. Of course, it's not like dropping skills is unprecedented - Elementals don't have a Master skill, for instance. You also appear to have bumped its speed up a little from Pathfinder.
Overall, it seems like a reasonable port given the system differences.
As mentioned above, cold iron still exists within Starfinder, along with alchemical silver and adamantine. The Starfinder entry for cold iron even mentions it works well against demons and fey. I would hope in a demon-focused campaign, a GM would make cold iron available.
The resistances come free with the demon subtype graft and in fact the Pathfinder version of the dretch does have the same resistance. However, looking through Alien Archive, the only CR 2 creature with any kind of resistances is the Kalo Sharkhunter and it only has one. So I reduced the resistances.
My initial thinking behind the stinking cloud mimicking the smoke grenade was that a smoke grenade is an item a 1st-level character can get his hands on for only 40 credits. The dretch here has spells because of the secondary magic ability, and per the table on page 143, it's only supposed to get "1/day - two 1st level spells." Stinking cloud is a 3rd-level spell in Pathfinder, and in a game with only 6 levels of spells instead of 9, giving this creature a 3rd-level spell seemed really powerful. However, upon looking at it further, it seems like the purpose of stinking cloud is to get melee fighters away from the caster and it doesn't do anything beyond that. And the creature would have to move out of the stinking cloud to pursue its enemies anyway. So I rewrote the ability to be more like the Pathfinder spell and gave it a save DC as if it were a 1st-level ability. And seeing how it's just farting, not provoking an attack of opportunity seems fine. :)
(2nd attempt)
DRETCH -- CR 2
XP 600
CE Small outsider (chaotic, demon, evil, extraplanar)
Init +0; Senses darkvision 60 ft.; Perception +7
DEFENSE -- HP 25
EAC 13, KAC 15
Fort +6, Ref +4, Will +1
DR 5/cold iron or good; Immunities electricity, poison; Resistances acid 5, cold 5, fire 5
OFFENSE
Speed 30 ft.
Melee Claw +11 (1d6+4)
Offensive Abilities stinking cloud (DC 11)
Spell-Like Abilities (CL 2)
-- 1/day — fear (DC 11), summon allies (1 dretch 35%)
STATISTICS
Str +2; Dex +0; Con +4; Int -3; Wis +1; Cha +1
Skills Escape Artist +7, Stealth +12
Languages Abyssal (cannot speak); telepathy 100 ft. (limited to Abyssal-speaking targets)
ECOLOGY
Environment any (Abyss)
Organization solitary, pair, gang (3-5), crowd (6-12), or mob (13+)
SPECIAL ABILITIES
Stinking Cloud (Su) Once per day a dretch can release stinking gas from various orifices in its body, creating a cloud of dense, yellow-green fog. The cloud has a 20-ft. radius and obscures all sight, including darkvision, beyond 5 feet. A creature within 5 feet has concealment. Any living creature that inhales the gas must succeed at a DC 11 Fortitude save each round it remains within the cloud or become nauseated. This condition lasts while the creature remains within the cloud and for 1d4+1 rounds after it leaves. This is a poison effect.
I think ER 10 was fine, but it's not unreasonable to cut it to 5. The new version of Stinking Cloud has no stated duration, though. I imagine it's intended to be either 1 round per level (CR) and should thus be 2 here, or is otherwise the smoke grenade's full minute. Either way, that should be stated.
It should perhaps also be stated that the cloud is created centered on the Dretch itself, but it can be inferred easily enough. Unless it can be created at a distance as per the original spell, in which case it very much needs to be clearly stated.