This guide is just a catalog of all the useful combo I've accumlated while playing demolitionist. A demo that combos correctly is to kill/decap the fleshpound/scrake without it charging. The guide assumes that you know the basics (ex. Reload Cancel) already. There is a few disclaimer and general advice that you should read before walking the true path.

Disclaimer
*All skills in the tree are on the left
*This is made for 6 player HOE
*These are the combos/knockdowns/stuns that I use and are not the only ones available

General Advice
*Learn how to reload cancel
*Try not to blind other players with nuke
*If you can't finsh off a big zed before he attacks, don't bother shooting
*A nade thrown during zed time will not stun since it is nuked
*A RPG Headshot on a scrake has a chance of not stumbling (*scrake will stumble if he is attacking)

*Scrakes
Able to stun scrake with nade when thrown at the pelvis, ass, legs
Able to knock down a scrake with a RPG shot in the legs

*Fleshpound
Able to knockdown unraged Fleshpound(FP) By RPGing the FP's pelvis, ass, or legs.
Able to stun unraged FP by throwing Nade at the FP's pelvis, ass, legs.

DBS -> Nade -> 2*DBS
https://www.youtube.com/watch?v=nAjIViFPsxQ

DBS -> RPG
https://www.youtube.com/watch?v=wwMQ02cldYE

RPG -> M79
https://www.youtube.com/watch?v=J1V9a2XuxTo

Magnum -> RPG
https://www.youtube.com/watch?v=XLyyfC0S674

RPG -> Magnum
https://www.youtube.com/watch?v=622uXtSdbvI

6*HMTech Pistol -> RPG
https://www.youtube.com/watch?v=Z4kUTliAlBE

Deagle -> 2*HXTech Pistol -> RPG
https://www.youtube.com/watch?v=NxKjox9VY-0

Deagle -> 2*9mm Pistol -> RPG
https://www.youtube.com/watch?v=ploHpiaO7uA

Deagle -> RPG -> Bash (Nico)
Disclaimer: Scrake has to be attacking for it to be a guaranteed stumble
https://www.youtube.com/watch?v=2NJVc3cG7Jw

9*9mm Pistol -> RPG
https://www.youtube.com/watch?v=b4aB8ne1ecE

Nade Stun -> RPG -> Upward Knife Slash
https://www.youtube.com/watch?v=IPQb0Oy94XQ

Body Bash -> HX25 -> RPG (Joint effort with Nico)
https://www.youtube.com/watch?v=vwwc-KGVHcY

3*M16 -> RPG (Walking)
4*M16 -> RPG (Dead)
https://www.youtube.com/watch?v=MATjZuiBPWk

3*AK-12 -> RPG
https://www.youtube.com/watch?v=meMbHGFKYA4

2*SCAR -> RPG
https://www.youtube.com/watch?v=5dnGcSP9yN4

1*Magnum -> Nade Stun ->6*SeekerSix
https://www.youtube.com/watch?v=hGu67KlBQyg

RPG Knockdown -> 2*RPG
https://www.youtube.com/watch?v=RmJUlqN9F-0

RPG Knockdown -> RPG -> C4
https://www.youtube.com/watch?v=YUUS5xI0GW8

Nade Stun + C4 -> (wait) -> 2*C4
https://www.youtube.com/watch?v=4Anu4ZDCv0Q

Nade Stun -> RPG -> 2*C4
https://www.youtube.com/watch?v=mDAXUFUoJiM

2*C4 -> 6*SeekerSix
https://www.youtube.com/watch?v=XP86FBf4mUo

Nade Stun -> C4 -> 6*SeekerSix
https://www.youtube.com/watch?v=wNLilUVTpo0

Nade Stun + C4 -> 2*M79 -> C4
Nade Stun + C4 -> HX25 -> M79 -> C4
https://www.youtube.com/watch?v=9W9FlojZb2E

Nade Stun -> Nade Stun -> 3*C4 -> RPG (FP Apart)
https://www.youtube.com/watch?v=nvWZbXe-UiU

Nade Stun -> Nade Stun -> 3*C4 -> 3-4 SeekerSix (FP Apart)
https://www.youtube.com/watch?v=J8U_fUvOcYk

Nade Stun -> RPG -> 3*C4 (FP Together)
*Note: Even if you don't stun both, you should be able to kill both without them moving a step
https://www.youtube.com/watch?v=F5Mtshj3jnE

Nade Stun -> 3*C4 -> 3 SeekerSix (FP Together)
https://www.youtube.com/watch?v=99kL-uQF97M

Nade Stun -> Nade Stun -> 3*C4 -> 1-2 RPG (Triple FP)
https://www.youtube.com/watch?v=A7Yn_yLQhvg