Draconic eyes should be saved up for Hime Awakening. Only when you have a huge excess of them would you ever consider those weapons, so that means you either whaled, or have seriously bad luck to not get any Himes that can be awakened. In the case that you can grab a draconic eye weapon, you don't MLB them by buying duplicates, but by using bricks.
They paralyze spam the boss while working as a team to prevent anyone from being hit. That means other spending time on damage, they don't worry about being hit. Also, it might be time to change Unions. As long as you are doing 3m iPP every union event, getting into one of the 50 top unions shouldn't be an issue. But in the case you don't want to switch Unions maybe due to how closely knit your union is, you could organize with them on how to efficiently push rank. Spend first few days farming grails, barely spending them. Use all 400 on the day you guys planned on pushing, and if possible have someone focus on paralyzing while others join him to help dps.
Regalia should be easy to farm, aside from events where people rather spend BP on non-raids(UE/Raid) you should have no problem hosting Light Ults and getting people to help. If we calculate your gains from never getting MVP or vMVP, you get 3 Regalias and 6 shards a day. 10 days nets you 30 regalias for a weapon, and 60 shards. 30 days means 90 regalias, and 180 shards which translates into 30 extra regalias and 30 shards carried over to the next month. I would advise you to only get Joan's weapon to LB1-2 and start saving up regalias if you are close to getting Shingen, because you would want her and her soul weapon. Having regalia on hand means you can grab her weapon as soon as you unlock her.
As for bricks, you may want to use them on Mich's lance instead and her weapon has an FLB(LB4) which gives it an additional 25 levels in stats, 10 more slvls, and adds a defender+. Also, since it is a lance, it is something you're more likely to keep further down the road if you can manage to get a Phantom Lance(from ragnarok raids) and 6 lances total(including phantom and soul).
If you haven't Michael should be your main priority for awakening.
For tower, you messed up once but now you know how it works. That experience means you'll have a much better time the next tower comes. Just know that there are a few things to consider:
1) If you can't complete all 3 objectives in one run, aim for the non-thunder hime objective. For example, run an off elemental balance/defense/tricky hime if you don't have one of the correct element.
2) You can run a floor that you find very hard with a completely off-element team, because getting the bonus from floor completion is better than nothing.
3) If you are stuck on a floor(too hard etc.), you can always go back to complete the 3rd objective for lower floors. For example, you had to bring an off-element balance hime for floor 8 to get the 250 medals, instead of giving up on the event now that you are "gated" you can go back to floor 8 and complete it again for the "only xxx characters" for an extra 150 medal+turn bonus. This would usually be done at the very end of the event knowing 100% you won't get the floor clear reward with the remaining attempts.
You have a very good Light team, to the point I'd like to say I'm awfully jealous considering gacha only ever gives me fire himes that aren't as useful. I did not know what himes you had, but now that I do I can tell you, investing 80% into Light, and the other 20% into an element to prepare for an event that requires said element(tower etc.) at least 3 weeks before hand will be enough for you. An easy way to recognize if you are prepared at a basic level for off-elements would be, 100% assault, at least 60% elemental from soul weapon+eidolon, and if possible 20-30% defender. More would be welcomed but not at the expense of neglecting your Light for a long period.
Another thing if you haven't already done so, level up your accessories for hp/atk. If you don't have as many SSR accessories, leveling an SR accessory is fine too but keep it around 25. Even at just level 20, 3 SRs provide about 1500 hp+attack combined for your hime.
If you were running Nandi with a Light team on AAB, note that it could be possible the challenge came from missing your debuffs. From what Slashley told me before, the chance to debuff(not accounting hime's personal hit rate with it), Elemental advantage>off-element>same element>elemental disadvantage. So if it was a problem due to debuff, just know that you will have a higher chance to debuff this time as your chance "upgraded to the next tier".
A Certain Kamihime Encyclopedia