I know shadow damage is key in taking down gremlins (The op acheron in particular) but is there any other sword that is good against them?
Shadow damage
Not directly effective in terms of damage, no. Look here:
http://wiki.spiralknights.com/Damage#Monster_Resistances
All monsters are neutral to normal and something else, resistant to something, and weak to something. The three specialised damage types are always defended against differently, with a few exceptions. Those exceptions include Battlepods (neutral to all), Vanaduke's mask (neutral to Normal, resistant to everything else), lit Oilers (like slimes, but resistant instead of neutral to Elemental). If you wanna get technical about it, the most powerful defence on a regular monster will always be two times the neutral defence on it. E.g. if a Slime has 50 Normal and Elemental defence, it will have 100 Piercing defence. These numbers are as far as I know not related to the lowest defence, e.g. the one Slimes have against Shadow.
If we're talking about plain effectivity, there's the Flourish line. The combo is very handy for bringing Gremlins down, but it's not very good for crowds that Gremlins tend to come in or the Constructs that often accompany them. There is also the Cutter line. Gremlins have a weakness to poison which means that they'll suffer an extra bit from it if inflicted. The DVS is also very handy for bringing down Mender shields, although you're better off not spawning it in the first place which you can avoid by using powerful attacks. Gran Faust and Acheron combos, notably. The Acheron charge can work for this but if they survive the initial hit and the explosions aren't powerful enough, the explosions will trigger the shield which isn't a very good thing.
If you're interested in guns and bombs, I could tell you about those as well. Better yet, Zeddy could tell you about the wonders of bombs! He's bound to read this thread sooner or later, I bet.
A well placed Suda charge in a group of gremlins will crush your enemies, show them driven before you, and let you hear the lamentations of their women.
... and also leave you open to attack, unless you do it perfectly. Loads of damage though.
Cutters, Winmillion, and Flourishes are all pretty safe picks, but I seriously recommend getting a bomb or a gun for them instead. More specifically, the Spine Cone bombs, or a Callahan.
I believe a lot of people favor Final Flourish/Barbarous Thorn Blade for singling out all gremlins pretty well as an alternative. Honestly, I wouldn't use Gran Faust against thwackers since you're left immobile for quite a bit if they get out of range for the combo finisher. Now if you wanna be Mr. Hipster, you could grab a Winmillion, Dread Venom Striker, Levi, or Suda.
Though gremlins are more easy to defeat safely with if you grabbed a Sentenza or Dark Briar Barrage instead of using swordplay, they're agile furballs that punish close range attackers easily.
well then, it's simple. We kill the gremlin.
I agree with the other posters here. Acheron, Obsidian Edge, and Gran Faust are the obvious choices, being shadow. But Final Flourish/Barbarous Thorn Blade is also pretty good. (It used to be better, before the range was shortened.) And really any sword can take them down. Even a Combuster charge can kill them fairly quickly, even though they resist the elemental damage and the fire. Shadow guns and bombs are also great, of course.
The only other thing I'd add is that gremlin thwackers (the ones with the big hammers/axes) are pretty nasty monsters. Three Rings intends them to be comparable to mecha knights and greavers, although I find mecha knights harder. Stepping into a thwacker's face is not very safe. So you will find them much easier if you use sword charges, rather than regular attacks.
Heavy swords are nice for getting some breathing room, and the charge lets you crush gremlins ( and everything) into a fine powder before they dodge or run away, or activate their CTR MAX MAX MAX MAX 5* Thwack Hammer.
Ach is 1st choice, but the poison from Obsidian Edge is super useful with healers around, cause they do half the work for you if you don't OHKO the gremlins with your charge but poison a crowd of them.
Covering what others have said, Barbarous Thorn Blade charge would be more useful against crowds of gremlins than the Final Flourish charge, but I haven't used either against t3 gremlins recently to have a good comparison.
Normal damage and flourish type weapons thanks for the advice, oh and for guns i"m going to wait for the update
There is something hilarious about poisoning a group of Gremlins and watch their Mender kill them all with AoE "healing".
I heard the Wild Hunting Blade can juggle Gremlins easily, more than the DVS.
DVS is plain better in terms of the combo (speed, poison), but WHB's charge is pretty okay if everything is facing you and nothing is out of range.
From what I have seen and tried I have two other sword I use against gremlins. My first choice after the acheron would be DVS, as it is fast and the poison nullifies menders. Another plus for DVS is that it can take out the mender shield very fast, plus the poison it is almost as if it was made for killing just menders.
My second choice is winmillion, it may sound weird as gremlin run away from projectiles but hear me out. The skittish behaviour of the gremlins when around projectiles rules mostly in your favour with the winmillion. The gremlins have a terrible habit of running infront of each disk most of the time, making the normally hard to hit winmillion into a six hit calibur. Note however that using this will most likely make your team mad at you as they will have a harder time hitting them.
Using things other than acheron or edge on gremlins tends to just put you in a handicap, although they might be more fun. If you want fun get a winmillion and swing away while solo, it is fun to see them jump into the disks. If you want effective get edge or dvs, both kill menders and make life a lot easier.