Assassin strike

Your SG buddy is full of beans. Run in, build up, hit AS and wait for the giant numbers. Make sure you're not in a position to get stomped afterward.

Out of all my toons, Arachnos Widow, Blaster, Controller, Corruptor, Dominator, Mastermind, Scrapper and Warshade, I would say for a Very High Damage, precise single target attack, this one on my Stalker does it best. My Blasters Nova is my best AoE weapon but for sheer single target attack, I can say that AS is very powerful. Before I normally use it, I would activate my Build Up (if it's fully charged) maybe pop a couple of reds (depending how tough the foe is) and then of course be in "hidden" mode which is what makes this power work so effective.

As paragonwiki describes it: "A signature Stalker attack. This attack does moderate damage on its own as a frontal attack. However, if it is executed while you are Hidden, this attack will do tremendous Smashing and Energy damage as you waylay your unsuspecting foe. This attack may be interrupted if you move or are attacked while executing this power."

I have been in mish'es where I was the one to sneak up behind the AV and use AS to take the initial blow. Of course, one blow on an AV won't take them out but it will sure help knock down his life bar a bit. The tricky part is after you strike, to get the heck out of the way for the tank to draw aggro back, otherwise you will have an AV chasing you around until you can hide from site and go back into "hidden" mode. You would also hope your teammates will draw some aggro if the tank doesn't do it right away.

I have also noticed a weird kind of phenomenon that sometimes happens with this power. I would describe it as a "wiff" where you hit nothing but air...lol. You, of course, can reload again once AS recharges and go at it again. The good thing is you will still be in hidden mode so you won't be exposed on that miss. The big key thing here is to aways try to be in hidden mode to get the most from AS. It is intended to be used best while hidden and to do the most damage. Lol, you know after re-reading your question again, I though the OP was the Stalker, then I realized the OP was the Brute and the OPs friend as the Stalker...heh..heh. The OP should start up a Stalker and try them out. And if anything, at least you know a bit more about AS right? Hope this helps

Breaking Line of Sight can and does interrupt AS after the animation has started, so if you're herding a mob around a nearby corner I can see how that would interrupt your bud's AS on occasion.

Autohit AoE's like caltrops and glue patches will also interrupt an AS every time, so if you're attacking foes like Longbow (Glue Grenade), Malta (Acid mortars) or Tsoo (Caltrops) - off the top of my head - and if you're not running to the opposite side of the spawn as your stalker then it's likely that the mobs that throw those powers are catching your stalker in their AoE, and even though the AS is animating it will interrupt it.

The most likely culprit, however, is you rushing in at the same time as your friend to attack the same boss. Brute players (myself included when I'm on my brute) like to start with the biggest bag of hitpoints in the spawn because we can let the minions wail on us to build up fury while we kill the boss, and then and one and two-shot the minions for dessert.

Stalkers also see the biggest bag of hit points as their prime target, so I'd be willing to bet that you're both charging the same target. Now this isn't something either of you should have to worry about - the cause of the problem lies in the game's movement mechanics. See, when a stalker rushes in to hit AS, we have two options with regard to getting to our target: hit F for follow (and relying on the game's engine)or whatever our favored movement key is (W, in most cases - relying on our own judgement) to get us into range and stopped long enough to hit the attack.

If your friend is hitting "Follow" to get to his target, tell him to stop it, because his target will immediately close on you when you enter, pulling his following toon along and interrupting the AS through the stalker's own motion.

If your friend is running in by holding down W and relying on his eye-hand coordination and reflexes to get him in ranged and stopped long enough to hit AS. What happens then is something that's beyond either of your control. Namely, his finger moves from the movement key to the AS key faster than his toon can stop moving, resulting in a half-second or so from the time when the movement key is released that AS will - if activated - be interrupted by the server, which thinks the stalker is still in motion. So the answer is to pause and wait for the toon to visually stop moving before cueing AS. When that happens, many times the target has moved out of melee range to attack you, the brute, forcing your stalker pal to relocate and try again.

The best advice for your friend is to open with another move besides AS, placate the boss you're punching and then build-up/AS while the boss is already aggroed to you. Situational caveats regarding to caltrops and similar powers apply. YMMV.

Sometimes they walk away after activation and it still hits them. Pretty comical when it happens.