16F is actually not very hard at all, but you need very specific information (and a calculator, but I'm sure you have one by now) to make her a cakewalk. I beat her with Reimu at Lv. 78, and probably could've done it lower if I wanted.
Key information:
-Very high (or complete?) PAR resistance
-Very low stat-debuff resistance
-Very resistant to SPI
-Very weak to NTR
-650k HP
-All her attacks are magic based.
-She can use NTR (Sakura Blizzard) and MYS (Piercing Light) attacks, but her most dangerous attacks by far are WND (Flying Insect's Nest) and SPI (everything else that's elemental).
-Uses Djinn Storm -> Barrier Change (DEF+MND Up 50%) at 400k HP, Barrier change changes her to her second form.
-Second form unlocks Objective Barrier (Multi-target SPI attack, lowers DEF and MND) and Charming Quadruple Barrier (Multi-target SPI attack, lowers ATK and MAG)
-Uses Djinn Storm -> Barrier Change at 200k HP. Barrier change changes her to her third form.
-Right after changing to her third form, she will use IN Quadruple Barrier (All stats Up 100%), then spam Mesh of Light and Darkness (strong multi-target SPI attack) and Hyperactive Flying Object(strong multi-target non-elemental attack). THIS POINT SHOULD NOT MATTER IN THE SLIGHTEST.
More detailed strategy:
First matter is surviving her. As said, WND and SPI are the attacks you should be most afraid of. I generally suggest using two tanks in your first two slots, and giving them the best SPI, WND and MND equipment you have, since quite a few of her attacks are row-based. After that, pick another 4 or so characters who can survive a magic onslaught and pump their SPI and MND up. Try to make sure the two tanks + 2 of these 4 characters are always at the front when Yukari gets a turn, so that you can take her magic attacks decently. I heavily suggest Reimu, Minoriko, Sanae and Ran among these 4, so you can keep them out and keep them buffing and healing everyone if needed, especially during form 2 when Yukari likes to debuff your MND with Objective Barrier. Have those 4 buff defenses, and bring Reisen along as well to nerf Yukari's MAG and SPD and you should have no problems surviving her attacks in forms 1 and 2. Yukari might do a move that heals all her stat-debuffs; if she does, just bring Reisen out again (carefully) to debuff her again.
Djinn Storm isn't hard to deal with if you're expecting it. When she's close to 400k HP, put a fresh Discarder on her and make sure she's -50% in MAG and SPD especially, choose 2 members who you think can recover SP the fastest/use little of it to function (Minoriko excels here), beat her down past the threshold, then bring out those 4 and buff everyone's MND before Djinn Storm goes out. After it does, focus like mad, heal/rebuff if you need to, survive a few more turns, Discarder her again once Reisen gets enough SP, and regain your momentum.
Attacking wise, as stated earlier, she's extremely weak to NTR. She also has very high MND, so most magic will not work well; Kaguya's and Patchouli's MND piercing can work okay if you must. Suwako is heavily, heavily recommended for this fight; it's very much possible to break 100k damage using Croaking Frog if you've been spending some of your bonuses on her ATK, and with some ATK buffs during the fight. Otherwise, bring your offensive powerhouses, preferably those who are relatively quick as well; Suika, Chen, Flandre, etc. But keep them heavily protected as they're important for the end; don't ever let them be in the front 4 when Yukari's about to attack, especially since during her second form, she can nerf your attackers' ATK and MAG if you keep them out. I also highly suggest only using one offensive character to throw attacks at her occasionally, and leave the other ones in reserve at all times, using Ran to buff their ATK.
If you couldn't tell by the above, the key part of beating Yukari easily is NOT LETTING HER BARRIER CHANGE TO HER THIRD FORM. Gank her HARD before she can do it. Ping her down to as close to 200k as possible, use Ran to raise everyone's ATK stats while she's soaking up magic hits throughout the whole battle, so that your attackers should have +60% (except whoever you chose to ping Yukari down) and make sure Reisen's brought her DEF, MND and SPD down to -30-50% (takes quite a few discarders thanks to her Barrier Change buff at 400k HP). When she's close to 200k HP, wait for her to use a long-delay move like Sakura Blizzard, then bring out all your big attackers (or Kaguya, who can make your best attacker go twice) in all your slots and blast her down before she gets her next turn. In my case, Suwako's Croaking Frog -> Kaguya's Buddha's Stone Bowl on Suwako -> Suwako's Croaking Frog again often did enough by itself; put other big attackers with ATK boosts in the other two slots should make it a certainty. And even if you can't gank her completely, you'll have lowered her HP by so much that you only need to survive and DD her slightly for her last form to beat her for good; and it also helps that she spends the two turns after Djinn Storm buffing herself only, giving you time to recover SP for a last hurrah.
If you follow this ganking method, you generally shouldn't have any problems and the fight is actually fairly quick and painless; I ended up farming her for her Gran Grimoire drop.