i'm not a very good instructor, so i'm just gonna write this as i go.
let's start with end-game stats, imo these are the 'perfect' end-game builds but ofc to each their own
*tutorial for beating the Lovers is at the bottom :]
notes: i play on normal, but i do tale on insane and have beaten it with this party, most of the time i don't even need to use turns positioning/summoning/soulbursting much since most enemies die in a few turns.
the lovers are crucial to nera's build, and the party mostly, for +energy and massive damage dealing/tanking for sisily. i can send sisily out into the front lines and conjure the lovers in front of her so she is able to participate but not be in danger. (if you are a donating member i would be happy to tell you how to get them!!)
my builds are mainly for damage, i'm not TOO much for strategy. i enjoy dealing massive amounts of damage and going full gung-ho, destroying enemies.
power: 10 endurance: 5 magic: 15 agility: 20
perks: enchanting rhyme (-25% enemy defence), absolute authority (+2 power, -25% enemy defence)
nera's build is a bit tricky and takes some preparing. you are to have absolute authority equipped, though with your base power + power overwhelming you would only have 11 power, not enough to qualify for the perk. what i do is play through the game like normal, (first raising her power on level 2 for power overwhelming then agility to 18 so i have enough leftover points to raise power more and endurance) and then at level 9 (once absolute authority is unlocked and equipped since i raise my base power to 10 + power overwhelming and i equip enchanting rhyme) reset her stats with either reset potion, since you get 2 in the game (one from beating lovers and one from pino/chio) and aim for the following:
power: 7 endurance: 4 (none added) magic: 14 agility: 19
each is lacking a point, i use all of the +1 stat potions on nera, since her build requires every point you can spare. she's the tank/main dps and both sisily and gelyan will be fine with -1 point from their builds.
(power from holy egg, endurance from forest memory ((since forest memory is late in-game i have enchanting rhyme beforehand on pre-build)), magic from crystal memory, and agility from beating the lovers.)
at l12 (which is obtained after you beat the second to last battle, the beholder 2.0) her stats should look like the above. i greatly prefer this specific build because she can achieve 40 power (the maximum) with no help from grand blessing/blue paradise, in 2 turns from moon waltz (15 + 15 + 10 = 40) and then can do 3 maximum damage rounds of crystal ballad.
with this build nera is quite squishy, but if you have fool conjured she should heal back her full health easily.
power: 5 (none added) endurance: 26 magic: 6 (none added) agility: 9 (none added)
perks: unshaken heart, rejuvinating heart
ridiculously easy. nothing but endurance is added, for maximum soul pact damage. sniper gelyan is very powerful, yes, but soul pact has high accuracy and higher damage/versatality overall + tankiness. at level 11 without the endurance potion he should deal about 9.7k per turn, for just less than 20k damage per combat.
power: 4 (none added) endurance: 3 (none added) magic: 28 agility: 10 (none added)
perks: energized soul, shining soul
same as gelyan, add nothing but magic. since nera has low magic for a good portion of the game (l2->power->lots of levels->agility->power->magic in order of priority) and gelyan as well, sisily is my battery and secondary damage dealer. warp tornado is still very strong, imo a sisily with a power over 8 or so is wasted potential in magic or agility. sisily can deal massive magicial damage (and with a specific strategy is able to beat the lovers as soon as level 4! in this build at least) and also provide the party with lots of energy. by level 10 or so she can fill the entire energy pool with essence of passion/energized soul. though fragile, usually i can place her diagonally by foes and she can deal damage and not be killed, since most ai's will either attack someone else or will not have the range to retaliate. overall sisily is the backbone and very important.
beating the lovers early-game
its almost impossible to activate the sequence to obtain them unless instructed how to, but at least for me once i found out i immediately wanted to see how early i could get them as they are a very useful tarot new-game. though they are very powerful, more often than you would think they are just an energy drain (only abilities are 5 and 8 energy each, ow) and the magician's energy transfer and -100+ resist on all enemies perk is more handy. they are not as broken as they might seem, it is perfectly possible to beat the game without them. you are able to engage the lovers very early, and you have to fight them to earn them as a tarot (for which you get a +1 agility item and a refresh stats item) but they are fixed at level 6 and 6.4k HP (regardless of difficulty setting) and may be a bit of a hassle to defeat.
the earliest i have been able to gain them is at level 3 (sisily, as you fight them with her alone) so here is the way i did it. don't be upset if you cannot defeat them on your first try, it takes several usually as you have a small chance of everything going just right for you to win. (that is, if you try to get them as early as possible like me. if you face them mid-end game, it is not difficult at all! probably easier than the fool's fight if fought at that time in the game.)
start of battle, move sisily to the lowest RIGHT corner of the map. summon the magician to your left to draw the Lovers to the bottom row. on your next turn, move magician to the square ABOVE sisily, and use meteor strike. do nothing with sisily. at level 3, this did from 1250-1550 damage, when i finally beat them it was only by a few points of HP so i would recommend trying to get above 1375ish damage just from the meteor. the lovers recover just about 400 HP every turn, and they have the same range and movement as sisily, plus in any engaged combat they WILL take her out. going head-on will NOT work in this battle, you have to bide your time and hope you get lucky. on your 3rd turn, move the magician two spaces to the left and engage the lovers. you are completely relying on them using forbidden flame twice, as that is the only combination the magician will survive in. (rarely the forbidden spark misses a few times and you MIGHT survive, but do not count on it.) use flame fury twice, and hope you hit for at least 1.1k each use (aka PLSDONTMISS). if by the end of the magician's turn, they are at 2,000 HP or less, engage them with sisily on her turn and use fiery flowers, you will act first. if the lovers use forbidden flame you will survive to your next turn in combat, and can hit them again with fiery flowers, hopefully taking them out because if not you are DEAD. if you do manage to defeat them, congrats! soulbursting magician may work, but the earliest i have had success with that as a tactic is level 4, on level 3 he can do (maximum) 3kish damage per attacking combat to the lovers, and the soulburst does maybe 1.6-1.8k damage, sisily engaging combat on her acting turn instead of soulbursting both 1, leaves the very low health magician alive (at the end of his attacking turn if he survies he will have very low HP and thus the lovers will attack him first if you had a 4th turn), and sisily can deal more damage engaging them herself at about 2-2.1k at level 3.
if you are trying your own strategy, avoid combat for as long as possible! if you hit them with meteor strike they will already have a good chunk of HP taken out before you even have to engage them. they have much higher health than both of you (sisily and magician) and will absolutely eviscerate you if you let them engage you in attacking state (100% of damage potential) so patience is key.
good luck and have fun!