Si techs are generally useless in multiplayer. Even a fully powered FOnewearl (30 techs, Simple Staves, Simple Merges) will take 3-5+ casts to kill enemies (these are the weaker ones), and compared to other options available to other FOs and other classes, that is very slow.
The replacement for techniques in Multiplayer are melee attacks with select weapons (including Demon's and Hell). Gifoie is useful to use as well for damage, as it has a lingering effect and can hit 4-5 times on spawn. This is useful for ganking enemies such as Satellite Lizards and Merissa A (only use Gifoie when a spawn with these enemies is coming up, don't use it everywhere).
With a FOnewearl specifically, you don't really have many options for melee. There's Demon's from Slicer of Fanatic, S-Rank Mechgun and Bringer's Rifle, along with Hell Raygun, and I suppose Vivienne for direct damage (although it's not too great). FOnewearl's versatility in multiplayer is kind of low...
Unfortunately using techniques in multiplayer in general is a very bad idea. Not only is their damage somewhat pitiful, you will cause mass "damage cancel" to the entire party and only slow the game down.
Are there certain attack combos better for activating my specials on my arrest/demons needles?
What about just damage overall?
Nope, just use combos as you'd expect them to be (NNS, NHS, NSS, HSS, whatever). The "SNS glitch" only works with weapons that give you time to start another attack before the previous projectile hits (easiest with Slicers and Cards).
