I'm a pretty experience GM in D&D 3.5 so I know how to run campaigns but something really has me stumped. In my Deathwatch campaign, I have a power gaming Techmarine player. His damage output is minimal but the issue lies in the fact I can't deal him any damage. His Armor is simply too high from bionics and his toughness was one of his first maximized stats. I don't want to pull a "GM says you take this much damage because he does" stunt because I really dislike doing that unless necessary. What can I do to stop this?

Also, I need to figure out a way to deal with that pesky machine trait, mostly just the immunity to mind affecting effects.

Finally, I just recently read the updated weapons errata and will be switching to that immediately. I plan on keeping the original rates of fire because I like them far more than the new ones. Is that a good idea?

FYI, Im relatively new to Deathwatch's system so we keep rereading rules as a group and figuring out how things work. My players are also Rank 3.

I would use the RoF from the errata. I don't really have a good reason, but I like all of the errata stats more than the book stats.

Easiest way to deal with the Machine trait is to say that mind affecting powers still affect a Techmarine with that talent. They started with a brain, and even though they are more machine than flesh, they still have a brain. In the fluff, even the most senior tech priests and techmarines, the ones that replace their entire bodies, can't replace their brain. It just can't be done (without heresy). Therefor, players will always be affected by mind affecting powers.

If you need something that can hurt a techmarine, start using enemies with high pen weapons, like Lascannons, melta or plasma weapons. Venom cannons work if you use Nids a lot in you missions. Power weapons tend to have high penetration as well. You could also use an enemy with extremely high strength bonus for it's attacks, like other Space Marines, or a Hive Tyrant, or a Carnifex.

Tau Broadside Unit with the Rail Guns will chew right through a player like that. if I recall correctly the weapon is 3D10+4 Pen 9. it's designed to be an anti armor weapon so it can do gnarly damage. or you can throw a twin linked lascannon at them.

I know the feeling with power gaming Techmarines. Mine isnt too bad and has become more of a team player as the campaign has progressed. He still has cybernetics all over and the machine trait lots of times. I think he can ping about 22 damage all in all.

I dont get around it that much but i do find hiting him repeatedly is effective at takin 1 or 2 wounds off a turn. As long as the baddies are of Elite level. If you really want to hurt though use a genestealer from the first mission of The Emperor Protects. They can attack four times on a charge with a WS of nearly 70. They have Razor shap and deal a hell of a lot of damage. One charge should be enough to kill a Marine(If that is your desire).

Also think about weapon traits, im refering specifically to warp weapon and the toxic qualities. These ignore armour which is always fun. Felling is also a nice trick to pull out. The look on a players face when they realise they are taking 24 damage at Pen 6 and there TB is 5 and not the mighty 10 they were expecting.

Personaly I dont like to kill players but it is important they dont feel invincible!! Especially the bloody Techmarines!

I generally havent found any issues with Techmarines, although my players are up against Tau and therefore have learened that Cover is King when taking on Tau Hordes and Elite units. Sure techmarines can take a massive pounding over some of the other Archetypes and the Machine trait does some amazing things. It's not all gravy however and can be balanced in some interesting ways. These include for the canny GM...

Eelectro Magnetic Emissions and Pulses, which can play havok with Cybernetics and Augments just as easily as regular gear.

Ferrous Reactive Acids

Scrap Code/Dark Mechanicum Code Infection for those tastey Corruption Points and Insanity points as well as system failures

Xeno-Tech Viruses for more of the above

Tech Heresy Subsystem Bypass and Overide of the Machine Spirit, who needs mental Psychic powers when you can pull THIS on your Techmarine players.

These are just off the top of my head, do a little Mechanicus research and I am sure there are many many more ways to play with your Techmarines in their own playground. Infact for further inspiartion I would recommened reading Mechanicum, Eisenhorn Trilogy and the Arbites based trilogy as well all of which have some very good looks at the Universe as a whole and covers aspects of the Mechanicus and Imperial technology at their level and what can be done with it.

Statting such things is not hard either. Just re-use an existing effect you'd like to present at your techmarine or come up with a duration table with a cascade of effects resisted by TechUse or Willpower ect of varying difficulties.

If you want to annoy a fully cyborged techmarine in power armor use haywire weapons. Techxorcism guns and EMP grenades will shut down his bionics AND his power armor, making him unable to do anything at all.

If you want to kill a fully cyborged techmarine in power armor use lascannon.