@Tat
Slicing Weaves: Countering incoming channeling attacks with flows of Spirit (that's easy enough), with a distracting "lash-back" effect if one of your weaves is sliced.
FS3 already has a defense built in - every attack rolls against the defender's defense skill. I'd probably just have a specific skill called 'Slice' and make defenses against weaves roll that. The lash-back effect would take a little bit of code, but I think you could just have a failed attack apply a -1 attack or defense (or both) mod for the next round with some ease.
Multi-flow weaves: Creating lighting requires strength in both Air and Fire (as I recall, and maybe Spirit too?). It gets even more complicated when you get to the idea that the requirements for the same weave may be different for male and female channelers.
This one is harder, but not impossible. Spirit Lake's magic is set up so that you learn a spell, and you have to have the spell's School in order to learn it. It could certainly be made to require that you have air && fire. That we haven't chosen to do so is more a matter of OOC system complexity than anything, honestly. That might be a thing to consider - is it worth it to be super faithful to the texts, if it results in a headache of a system, or can you simplify?
Of note, if you were doing it the SL way, you'd create weaves that do specific things (for us, spells), because you have to config/code them all. If you wanted things beyond those weaves, you'd need some rules for rolling and what those rolls mean.
Shielding duels: Just straight up channeling strength contests, straining to cut the other person off from the One Power (magic). If the other person isn't holding the One Power, it's easy, but if the other person -is- holding the One Power, it's relatively impossible unless you a) distract them, or b) are much, much stronger than them.
So again, FS3's attacks are contested rolls. A skill called 'Shielding' or 'Duel' or whatever is appropriate could just do this as-is. Actually, not even in combat - just do a vs roll. If they don't have the One Power, their stats would be super low. Or mod your rolls. Etc.
ETA: Or magic One Power - we made magic an attribute that governs our schools, you could do that as well and then just roll your raw power for these duels.
Severing (hey look, I've thought of more!): It's Shielding with an edge (literally). If you succeed, you cut the other person off from the One Power forever (for certain definitions of "forever").
Same as above, but with GM intervention to adjust skills as needed on the result of the rolls. Though @faraday generally holds that FS3 isn't built for PvP, so that's a thing to keep in mind.
Overchanneling: Channel too much and you'll get really, really tired. Channel even more, and you might burn yourself out and make yourself unable to use the One Power ever again (for certain definitions of "ever again").
We have a mechanic like this in SL, but it's more RP flavor than something accounted for. That said, it would be pretty easy to add in a counter that ticks up every time you cast/weave something, and ticks back down on a cron, and if you hit a certain level it tells you 'you don't feel so good' and/or creates a job/sends a mail. The primary difficulty here is when IC time doesn't like up with RL time (like you've paused a scene, or fast forward a scene), so you'd want a way to set things manually, too.