Fair enough!
Speaking of spending on water on prints...
Are there any I should be grabbing first? And do I spend enough to MUB or just snag an assortment and build them up from there? I've got 55k water and wouldn't mind grabbing a few good prints.
Resounding Rendition is the jack of all trades option that pretty much all non-healers can use, with it being best in slot for many of them. Skill Damage is one of the most powerful Wyrmprint options, and Resounding Rendition pairs that with a powerful Crit secondary ability.
For weapon types that are more reliant on Force Strikes (Sword, and to a lesser degree Axe/Bow), the Force Strike +40-50% is also strong. The newest featured 5* Wyrmprint is deceptively powerful, pairing a decent Force Strike boost with a banked Skill Prep+75%. Other options include 5* Stellar Show and 4* Fresh Perspectives. Avoid the rest, for now.
Swords want 5* The Shining Overlord, Axes want 5* Kung Fu Mastery. Although 5* Dragon and Tamer is decent on Lances, it doesn't significantly outperform Resounding Rendition, so it isn't a priority.
For 3*, Dragon's Nest is a good option if you want a small or fast upgrade to 2* Dragon Arcanum. Get used to wearing Shapeshift Prep+10% on at least three characters during Raid events, and possibly the upcoming Astral Raids as well. Its the single best way that a newer player can contribute to a veteran player's damage output, since the Prep is shared among all teammates. The long term goal is probably to own three 4* King's Countenance and one 5* The Bridal Queen, but as a new player that will cost way too much Eldwater and you can get by just fine with the lower rarity replacements.
If you make frequent use of Elisanne or Melody, 3* Bellathorna and 4* Halidom Groom are strong options. As you work through Mercurial Gauntlet, try to find Elisanne Helpers who wear one of these and one Shapeshift print.
In general, I would recommend max unbinding each print before moving to the next. You want your player-controlled character to do well in Co-Op, before worrying too much about the quality of AI character Wyrmprints - they can get away with wearing just Shapeshift Prep + Anything for a long time. But there are plenty of players who would disagree with me on this point, so its personal preference. Do not, however, worry about getting your MUB Wyrmprints to Lv100 until you are very secure with your surplus Water - the cost to level is exponential, so stop somewhere in the Lv40-80 range for now.
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Regarding Mercurial, keep in mind that the levels are a trick - Wind Roy has the lowest HP per level, and Flame Roy has the second lowest. Don't over-invest to clear these, because harder elements are coming. Renee looks broken for Flame Roy, considering that Elisanne is likely to eat up the slot that you would want to run Jakob in. Speaking from experience, AI Renee is already busted in speeding up Flame IO clear times, and you don't even have to worry about her wasting Bog on chaff. Because Bog is so good, you'll want to consider not running Freeze characters (Lily/Fjorm/XCleo) if you can help it, though Fjorm is busted enough that she might prove to be an exception if you're on the Last Offense plan.
For setups with Ranged characters, wait to do Mercurial Gauntlet until the end of the month. They're losing the penalty to short-range damage, which could significantly help the AI's damage output against Roy.