Fleshpounds will rage if they take more than 360 points of damage within a two second period (remember that they have 50% resistance to the vast majority of weapons). Incidently, roughly a fleshpound takes a step every half second, so deal <360 damage, wait for him to take 5 steps, then hit him again.
They will also rage if they have line of sight to a player for more than 10-15 seconds (random number).
So if you want to take them with a non-zerker/medic class, you'll need to keep ducking around corners to break line of sight, and only hit them with small weapons infrequently so they don't take too much damage too quickly.
Scrakes will rage when they get to a certain % HPs, and will continue to rage until death (I think crisping them with fire might break their rage, but it's not a good strategy at all).
Generally, most kiters leave scrakes until the end of the wave to deal with, because assuming you don't rage them accidently, generally they're pretty harmless as long as you keep them at arms distance.
You can flinchlock scrakes with primary fire on Hard or below but only as bezerker (need the attack speed bonus I believe). On Suicidal+, you can only flinch scrakes once, so this obviously becomes impossible.
Just practise with the M99 ^_^ You'll get there.
My favourite classes to kite with are:
Sharpshooter (loadout = crossbow + katana + MK23. Crossbow is now uber as well and can take out fleshpounds without a problem even on HoE, but is also light, quick firing and ammo is plentiful enough to take out sirens. Katana is just an allround useful weapon when kiting, allowing you to kill things at short notice and move quickly. I'll generally have a melee weapon in my hand when kiting for speed, so katana makes this fun. Mk23, as much as I dislike it, is dead cheap, light, has a bunch of ammo and although the 9mm pops heads off real easily, the mk23 has just that bit more punch to kill zeds rather than decap them which is useful if you're aiming to run through where they are in a few seconds.)
Support specialist - (Pump shotgun, Hunting shotgun, EITHER machete + handcannon/M79 OR katana+Mk23 OR axe.
Pump shotgun is so ubiquitous and useful it's untrue. I love it to pieces, despite the new-fangled shotgun. Only trouble is, don't get caught with it empty.
Hunting shotgun is almost unparralleled at close range destruction. Especially because you never need to reload it. It can also 1shot scrakes with the right combo, and help you take FPs with the right combo.
A melee weapon is essential for killings crakes with the HSG combo. Size of melee weapon depends on how much dosh I have, whether a berserker will buy me a katana, or whether I'll use it outside the combo as well.
M79 is a lot of fun as well for taking crawlers and little things and generally having fun. Handcannon I now realise is probably outdated by the Mk23 (old loadout).
Berserkers normal and (IMHO) best loadout is katana/axe/LAR, allowing you to kill any zed in melee in the game, and the cheap and handy LAR lets you bodyshot crawlers and 2headshot husks and sirens.
An alternative is katana/axe/M79/Mk23. The Mk23 lets you 2shot bodyshot crawlers, although is a bit duff on sirens and husks, but the M79 I am really having fun with lately. Plus, if the map has a lot of doors, you can kill FPs really quickly with it, rather than using the axe.
Axe stuns scrakes on headshots at higher levels, and is used for killing FPs.
I don't play medic/commando, firebug has no real loadout combos (flamethrower+MAC10/katana I guess), and demo is a bad class to kite with.
My general demo loadout is M32/katana/MP7M to give me a CHANCE to kite though, along with being the ultimate support class 8D