I do believe this guide needs a fair bit of an update, including suicidal difficulty for one thing, giving better advice as another.
Here's the SS's role:
Weapon loadout: Single 9mm, Dual Handcannon (for particularly open maps single is also perfectly viable), Crossbow
Clots - low priority - you should typically ignore them, they aren't your problem. When you have to shoot them, pop their head in one with the 9mm, if a group grabs you, panic fire with the dual HCs to extricate yourself fast.
Gorefasts - low priority - again, not your job to deal with them. Again, pop heads when necessary with the 9mm.
Crawlers - medium priority (high w/o commando) - commandos should be dealing with them, but your class is also suitable. One shot them with the handcannons, their headshot box is erratic and since you have a plentifully supplied weapon that can oneshot, do so.
Bloats - high priority - very dangerous on suicidal, these will kill if they get close. However if you pop their head with the 9mm from range, threat gone.
Stalkers - low priority (medium w/o commando) - not your job to deal with again. Use the 9mm on their head, if you have difficulty picking out their head, use HCs.
Sirens - *top* priority - these buggers define your role, let one of these past and your team may well be dead promptly. Use the crossbow before they become a threat, one shot and they're down. If one gets the drop on you, whip out the dual HCs and let rip.
Scrakes - medium priority (high w/o beserker) - they're definitely within your role to deal with, but the fact that beserkers can easily deal with them makes them less of a threat. Learn where their headshot box is, then one-shot them every time with the crossbow.
Fleshpound - low priority (top if they're after you) - if it isn't after you, ignore it completely. Seriously, your team is more likely to die from the zombies advancing under the cover of the FP than the FP itself. Keep killing the zombies you're supposed to be aiming for. If it's after you, get in a quick heal if you need to, then get out those dual HCs (if you have single HC, use the 9mm instead for its rate of fire). As soon as it starts an attack or taunt animation next to you, unload on its head. You want a stream of lead to be hitting it as the sweet spot appears. Its head will pop off, you can gtfo. This works even if some idiot enraged it.
Long story short, you should be using your 9mm most of the time, it's an easy headshot generator with plenty of ammo. When you see a siren (priority) or a scrake (for sport) whip out your crossbow and eliminate. If you stick to those with your crossbow its 40 ammo is plenty. Eliminate crawlers with your HCs, saving their ammo for emergencies usually.
THAT is how you play an SS.