Hmm. So it seems with Takedown, although it is worded really weirdly.
You can certainly Feint with Grapple, however. The Grapple Action listed under Attack Actions is the action you take once you are in a Grapple. Grapple itself is something you can choose to do instead of dealing damage with an unarmed attack. So, you're still in pretty rough waters: succeed on an opposed WS check, then make an unarmed attack againt an armed opponent (-20 WS), which cannot be dodged or parried if the Feint suceeded. Then, if they have a reaction left, they can make an Agility test to avoid it. You're not always much worse off if the Feint fails, because they may use their Reaction trying to dodge or parry the unarmed attack. On the other hand, the test to avoid Grapple is a pure Agility test, so they don't get their dodge bonus if they are forced to use it. Also, their extra reactions from Step Aside and Wall of Steel can't be used to escape a landed Grapple hit, so there's that.
Disarm is a fun option, and a pretty fair contest all around.
Or, if they disengaged and moved under charge range, they just so happened to be PBR for a full SA shotty blast, which their dodge should bring down into the survivable range. Another option is for the enemy to unload two FA autopistols PBR at them, which is pretty similar: lots of small bullets, damage mitigated by dodging. Or just an autogun. Pure melee NPCs such as Khornites can be given fun bonuses to make up for their lack of ranged inclinations, since normal melee characters have at least enough BS to make it a bad idea to let them have a full action at PBR.
Feel free to set up terrain which allows an NPC to Manouver the Assassin into a position in which his back is to the wall and the only way out is through the melee guy.
If Parry from Wall of Steel is part of the problem, enemies are more than welcome to Fast, or better yet, Flexible, weapons. There are lots of weapons which are both Flexible and Shocking, so once his Dodge reactions are up, those could be nasty. Snare and Toxic weapons are also an option, each of which has side effects once a hit lands which are harder to negate than simply making a dodge test. Smoke weapons, likewise, throw down a cloud which subjects anyone inside it to Suffocation, which is a Toughness test.
It's tricky to pull off, especially if the Hypothetical Assassin has high perception, but if an opponent who he is not aware of pulls off a successful Silent Move check on him, he may take any attack as if it were a Feint (from the Silent Move entry in the IH). The Blind Grenade throws down smoke which also blinds even sensors from photo visors. For a wider, longer-lasting variant, there is the Choke Gas grenade. The Stun grenade forces a Toughness test from anyone in its blast of 3 to avoid being outright stunned for 1d5 rounds.
I suppose if he blindly charges in, you can punish him by having the opponent throw down a Hallucinogen Grenade within 10 meters of him. Then he has to make a -10 Toughness test or have a 50% of tripping his way through the next 1d10 rounds.
And, if all else fails, Sorcery and Psyker Powers aren't dodgeable.
Hmm, despite all this, it would be nice if there was a weapon which imposes penalties on dodge the way some do on melee. Also, I still think that wearing heavy armor and carrying heavy weapons should come with a dodge penalty, the way heavy armor now imposes a concealment/silent move penalty.