What's a weapon combo?
Exactly what most people know and love. using one weapon for an set of attacks until it resets.

Some examples:
2 swings of Gran Faust or Divine Avenger.
3 swings of a brandish or flourish.
3 polaris shots.
etc. etc.......

this to many people, is known as spamming. unfortunately, some people don't know another way to fight.

what's combo weapons then?
a bit more complex if you want the short answer.

First you have to realize what many veteran SK players know. if you don't use a full combo on one weapon and wait a second or two, then you can attack faster over all.
the best examples are with guns. using just one shot of a Callahan or Alchemer/Driver and then pausing will get you around the rather long reload time and allow you to continue walking in between shots.

if you can combine this fact with swapping weapons then you get a set of combo weapons.
(warning: this can require some fast paced weapon swapping, on slower/laggy internet results can't be guaranteed)
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My favorite combo weapon set
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a Glacuis, Argent Peacemaker, and a Cold Iron Vanquisher. it's a set for vana runs in case you couldn't tell already.
if you care you can swap out CIV for a Levi, i wont be offended.... much.

Before you even think it. YES, you need then in that order for My style to work. (just trust me on this one)

1) the combo is set up with attacking with Glacuis then pushing the enemy away with a quick shield burst. (a great time to swap your weapons!)
2) follow up with AP for some quick safe damage as you close the gap on your target, and possibly stop Zombies from breathing on you. (it's just creepy and you know it)
3) now use CIV to attack or set up your charge attack to make room.
4) ---RESET---- a quick swap to Glacuis gets you back to higher damage and provides a charge to lock enemies in place. (for some variety keep going, or you can just skip the charge and start over) :D
5) if they're frozen you have PLENTY of time to swap over to CIV for a charge attack.
6) they're knocked back, so pull out AP? sounds good to me. hmm... i really like freezing though.... kite for both! >:3

if CIV sounds out of place to you, that's because it's there to help me with the Trogens. Bait while charging, dodge and unload on their backs. i know i'm making it sound easy. (insert practicing cliche here)
AP on trogens can be good for grabbing starting agro if you're the one who wants to fight them. also good for when you're not given the chance to charge but have an opening to attack.
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some other combo weapon sets i've found good are:

2 shot Polaris to push where i want shock to hold enemies for...
1 shot Plague needle for some damage and (crosses fingers) poision, on order to...
Glacuis for maximum damage (10% damage boost when attacking poisoned targets)
Glacuis also gets me close so i can shield bash to make room and make sure i'm attacking at Polaris's large bullet range. (rinse repeat for the win)

for LD i don't use a flourish, and as such i had to make up some way to beat people. it's hard, but every kill is that much better.
i use a GF, polaris, and AP (or Callahan for more dmg and more skill. also adds another possible status effect for some easy kills)

GF for initial attack.
Polaris to lock down escape paths and shock. (also used to block choke points)
AP (Callahan) to tag people out of flourish attacks and chase down runners. (both AP and Callahan give micro stuns on contact.)
after tagging, back to GF for the damage.
starting to see why i like it yet? :D
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if anyone tries my combo's, Please respond and tell me about it!
also tell me any combo's you think are good, or tell me how to improve these.

(in case you really can't tell by now... i'm a sword-gunner combo myself, but to all the bombers. <3)