Yda is extremely unique, but the most important thing to know, is that you want to open with her LD, to get her LD buff. From then on, if an enemy is unbroken, point Yda’s HP+++ at them (provided the orb is being maintained). Breaking an enemy with HP+++ gives her a free instant turn AND queues her next turn right after (this queued turn isn’t free). It also extends her buffs, so you never need to worry about upkeeping buffs from any of her other skills. See below for infinite turns.
On her free turns you can use her other skills:
Snap Punch is best used after her HP+++, so you have the hard hitting + version. It’s a single target shave and gives her some strong generic buffs
Touch of Death is also single target, dropping her poison, sap, & hp poison onto enemies (using 2 debuff slots). This has 100% splash, so it can be good in AoE situations provided that Snap Punch+ isn’t capping. Also good if she’s being targeted as there’s a big BRV gain afterward.
Tornado Kick is just hard hitting AoE BRV + single target HP. Big damage against a single target. Also a BRV gain here.
For infinite turns, it’s fairly straightforward to setup. A typical team looks like:
Yda, open with LD, spam HP+++ on normal turns, skills on free turns
Sephiroth BT, open with BT and stand there
Off turn damage from Vayne BT+, Kain, Garnet, etc. Cor won’t work as he will break the enemy before Yda, and CoD won’t work
Optional Debuffer to drop DEF, BRV/HP DMG taken up debuff and get swapped out right away for one of the above (not Yda)
Use calls to amplify damage further. If Yda is not capping her HP+++, reconsider your setup!