I spent a while last night solo practicing my parries on scrakes and fleshpounds and it seems that getting in an attack while they're stumbling will reset them out of the stumble when it lands so they'll immediately go into another attack which you can chain parry off of if you're quick about it, although I think there may be a damage threshold you have to go through for the attack you throw at them to actually cause the reset but only happens in certain cases so there's not really anything to worry about if you're playing a proper zerker with the pulverizer/shovel (katana bodyshots on a HoE fleshpound does not reset a fleshpound's stumble sometimes.) So basically parry > one swing > parry > swing, etc.
Getting to more advanced chaining, scrakes will stumble from an altfire hit of the pulverizer and will reset off of a normal swing while they're reeling from it letting you get in more attacks per parry. Parry > alt fire > light attack > immediate parry on their retaliation > alt fire > light attack, repeat.
Fleshpounds on the other hand will immediately rage off of an altfire so I still have to figure out how to deal with them raging as it's a lot more random (they have a couple of possible things they may do upon you raging them, such as immediately going into a swing or start the groundpound animation,) however getting a successful parry while they're enraged will reset them back to yellow and thankfully you won't have to do it too many times as the pulverizer makes decently short work of FPs (in my solo experience anyway.)
As for approaching either, they both have certain attacks they almost always do when you're just barely out of reach. Scrakes will wind up a slightly delayed swing so your parry will have to be timed for the delay, after which you can go into your chain. Fleshpounds, again, are only a tad more random. FPs can do a jump attack at you (this you cannot parry, it will not stumble them) so I recommend back pedaling (with your sprint key) to bait it out before going in, keeping in mind that they may also swing at you instead of jumping (he has a delayed right hook and a left hook that goes into a combo attack) but back pedaling should keep you out of the range of either.
A disclaimer though, this was only from one night of practicing in a solo match, so I can't really say if teammates attacking the thing you're trying to chain parry has any effect in the case that they do accidentally reset the stumble before you can get in a swing.
Edit: After playing some multiplayer matches, I can confirm that damage from any source will reset the stumble once it goes past a certain threshold. So depending on whether or not your team trusts you enough to solo the big guys, you may or may not have enough time after a parry/altfire stumble to get in a swing (depends on your weapon speed anyway.)
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