I won't get into the various justifications or counter-arguments involved in letting a space marine run along with starting level humans. Its your game so you should run whatever you want, however you want. If I was a non-SM player at your table though, I would expect some kind of compensation to balance things out...and although the nailgun sounds neat I'm not sure I would be bought so cheaply =P.

On the topic of damaging your SM, I highly recommend you use tools that already exist. Specifically, the Sergeant orders and the Las-gun expertise/mastery talents from Hammer of the Emperor.

If you combine the "Get Them!" order (+3 damage) with the various lasgun barrage and volley talents, a squad of dominates armed with bog standard lasguns on Overload setting can start putting out like 1d10+12 with 2 AP damage per hit. That should make even a space marine cautious.

If you combine the "Get Them!" order (+3 damage) with the various lasgun barrage and volley talents, a squad of dominates armed with bog standard lasguns on Overload setting can start putting out like 1d10+12 with 2 AP damage per hit. That should make even a space marine cautious.

Yeah.

You still have the overkill problem here though; that the human characters will be obliterated by such attacks, and that there is no reason for enemies to target the marine over them.

Well, that is part of the problem with running a group of such differing power levels.

Although I would consider the SM similar to an Ogryn (their defensive stats aren't too much different after all). Their size is the same, so they would both attract a great deal of gunfire and enemy leaders would definitely target them with co-ordinated salvos in order to bring down the threat as quickly as possible.

Obviously for maximum killing power they would be doing the same to the regular humans, but I think its thematic that the Sergeant would mostly use his orders on the perceived greatest threat. Its worth noting as well, that using these orders technically fuses together a lot of single attacks into one massive attack. Having a fireteam of five dominates essentially make one attack significantly decreases the number of shots being taken against the rest of the team.

It has the advantage that so long as the SM is under heavy fire, the rest of the squad should have greater freedom of movement and perhaps even be able to take down that pesky sergeant/junior officer making their lives difficult.

Also, these are just tools to keep everyone...including the SM...on their toes. Knowing that the enemy has the possibility of doing this kind of thing is psychologically quite potent. It would also be pretty logical if the dominates are seeing their lasguns just bouncing off the guy that their sergeant would step in and tell them to concentrate their fire...but up until its proven necessary they are operating their normal drill (IE combat as normal).

Or you could just have some major enemy know about the space marine. He is supposed to be secretly moving with the Guardsmen for some clandestine reason, right? Well make it so that the reason is some major enemy psyker or chaos marine who KNOWS about the other marine.

Whenever the enemies seem to be concentrating on him you can give some veiled hints that the enemies are looking for some particular person whose description fits him ("Looks like a human but the size of an ogryn. Supernaturally tough and strong, use your biggest guns.") and that would justify the extra attention he gets.

It would change the plot but at least it is some justification for the enemy attention he gets and tougher opponents.

Well, the OP has stipulated that the Space Marine here just looks like a really big guy, and the enemies (who do not know how much damage they rolled after all) are likely to chalk up his being hard to put down to bad shooting.

My advice may be similar to others, but in essence it is:

1. Weapons with Felling: Classic easy to pick up example is the longlas, triplex pattern lasgun brings felling and proven (5) to the mix, meaning it will always do at least 8 damage and ignore his UT.While for the average guardsman this isn't as scary, they have about 7 soak on average (3 TB, 4 Armor), while an armorless SM would have 4+4 TB, 3 AP for the flak cloak, with their TB gone, they start getting hurt easily. IIRC it may have variable mode too, which could let you pump up the damage even higher.

2, Make him a scout like others said, it makes some more sense, he wouldn't be as crazy as a full SM but still more than a notch above a human. Carapace would push him back up, but AP is easier to ignore than TB. Bonus points if he doesn't wear a helmet for snipers and other specialist weapons.

3. He WILL be the biggest scariest target out of your squad, so feel free to put in 1 bigger deadlier weapon and let them focus just on the SM, without armor a lot of weapons become even deadlier. As others said, the focused fire lets the rest of the squad move around and can even start doing a bonding experience as they start to work together.

4. One problem with Scout is Astartes Weapons, unless you keep him from being resupplied easily, which forces him to make each shot count.

The triplex pattern does NOT have a variable firing mode.

This is important because it makes it inferior to either a normal lasgun or long las (in addition to the lack of Felling and Pen). The triplex's advantage is versatility, which it pays for by being inferior in each specific mode.

(There is a long discussion of this in the rules section somewhere.)

Also as the OP pointed out, the SM will not be the biggest scariest target. He's just a large very well-muscled Guardsman, to all appearances. Enemy fire will be drawn to whoever is dishing out the most damage, not him.