For information on Treasure Goblins, take a look at this previously asked question.
From my testing, there is no other way to effectively close the portal besides interrupting the cast and/or destroying the treasure goblin. In Hell mode, all enemies have increased resistance and increased HP. Therefore, you HAVE to output more DPS to prevent him from getting away.
From what I have noticed, once his portal is down, you have a set amount of time to kill him or make him run away from the portal before it doesn't matter: He will get into the portal and your loot is gone.
A few recommendations:
- Try cornering him. This will obviously try to prevent him from running away and allow you (and your team if applicable) to dump all of your DPS in a concentrated area.
- If you have a mercenary or other teammate(s) with you, attack him from both sides - in attempt to not allow the treasure goblin from running away.
- If you have not attacked the treasure goblin yet, DON'T ATTACK HIM UNTIL YOU CLEAR THE SURROUNDING AREA. He will only run away once you start attacking him and if he, you, or any of your teammates aggro more enemies on the harder difficulties - it will get harder -practically exponentially per enemy - to take him down before he gets away.
- Lastly, and obviously, make the treasure goblin your party's main priority [if applicable]. Notify everyone in your party that you see a treasure goblin and wait until everyone is ready to take him down (and the area surrounding him is clear).
I actually find slowing/snaring them down more effective as it lasts and lets you run to position yourself to chase them into a corner.
I've tried this quite a few times, works really well because once they're in the corner you can start dealing constant dps towards the fiend itself because all they'll do is try to get into you (and they cant run further than that corner they've cornered themselves into).
For wide open areas, you could always try to nuke them (if your damage is high enough) and acquire a fast kill before they run into a mob of monsters or champions. Also, you could clear the entire area around the goblin out first before trying to take it (the goblin) out, also improves your chances of killing it that way. (I do this on inferno so that i don't actually have my butt kissed by mobs of monsters or champions while i'm trying to nuke that sneaky running bastard)
If you're talking about confusion/knockback/etc, i wouldnt recommend it as confusion (although it lasts only a while, only serves to make the goblin run away from you - which is what it exactly does even when not confused), while knockback pushes the goblin away, which isn't exactly what you want. You want to have a skill that allows you to catch up with the goblin and position yourself into a place where it will run into a corner or at least not into a fresh group of mobs (That's the basic idea, unless you're playing on a very easy mode).
Also, for the amount of reward that a goblin can give you, treading carefully and making a very precise action plan would actually be quite rewarding at times.
So yeah, its hard, sometimes confusing, but if you perfect a plan you'll be laughing all the way to your stash. :P
Best Answer
It does not seem to matter how fast you beat on them to determine the quality of the overall drops. The most important thing is to actually kill them before they open a portal and escape. The last drops upon death (and possibly right before death) are magic items, as opposed to gold, so it is well worth making certain you kill them.
There are different types of treasure goblins, and they seem to have different qualities of loot.
I encountered a Treasure Pygmy during Act 3 of normal, and, despite not killing him particularly quickly (certainly not as fast as I had killed some early treasure goblins), I had the extreme fortune of having 3 rare items drop from him. I'm not certain if this is simply an extremely lucky coincidence, or if the quality of the drops was influenced by the particular monster title.
Edit: I just encountered a Treasure Goblin much later in Act 3, and he dropped 4 regular magic items (no rares), even though I beat him down much faster than the Treasure Pygmy. This seems to corroborate the idea that the Treasure Pygmy may simply be a rarer, better version of the Treasure Goblin.
Edit: After quite a few more tests, ranging from chasing the Treasure Goblin (of a number of varieties) around, to beating them down before they could move more than a few yards, to killing them moments before they step through a portal, there is absolutely no discernible difference in the quantity or quality of items they drop. They always drop 1-4 magic items, plus gold, gems, and crafting pages/tomes (if you are high enough level for gems and/or crafting pages/tomes to drop).