So finally into maniac and have my lancelot at about 11k power - guess I can start grinding a bit now. From here, is it basically farming sigils and leveling those up? I'm almost capped on my attack mastery.
If you scroll up a bit you'll see the discussion but you NEED to get damage cap sigils, 4x V if possible although you can't really farm V before the end of maniac(on Angra Manyu, you'll spend a fair bit of time there). Second thing you can and should probably do is farm the fire bosses, the firewyrm and vulkan bola as damage sigils mostly use fire mats, and you'll need a ton to upgrade your sigils(upgrading dmg cap V to 15 is also high priority, eventually it'll be "wasted" when you get V+ but they only drop from like knicknack transmutes and the last boss so it takes a while to get them and they need to be the right ones too). Can wait for proud farming though.
There's also farming money/mastery points which can be done pretty well in the Slimepede quest(first one in maniac), you'll want mastery points for overmastery(and also for finishing your chars 100% trees, or at least mostly 100% as long as you take all the good stuff) and money well you'll need some for upgrading sigils and weapons. There's also an additional grind once you get to the end of Proud difficulty for the Terminus weapons.
I didn't grind too much in maniac personally, just went and did every quest once, then grinded for like 15-20mins(I got lucky on dmg cap drops though) and then moved on but ymmv.
If the Vizarak Firewyrm(?) is available, farm that once you've cleared out the Maniac quests. You'll get V / V+ (Dmg Cap is on its list), 1 Curio each kill, and weapon leveling materials. Save however many Azurite crystals you may have still, only use them on Dmg Cap V+'s. I hope you've forged the Ascension weapon and have it leveled to 150, if not, do so. Then start the Awakening process for it, once fully awakened it will be your BiS. It also gives a special perk called Sigil Booster, which gives +1 lvl to every Sigil you have equipped. Once you have that weapon fully awakened, start getting all his other weapons to 150 so you can unlock the Mastery perks for them.
As for Mastery, max out both Offense / Defense as you'll get more sigil slots. Make sure to also do all the Fate quests for Lance, as those will give sigils slots and stats.
This is where I'm at now after farming up stuff:
Even with all those Dmg Caps, I'm still capped on dmg, so the only way to do more dmg is getting gear which extends Dmg Cap, like the Terminus weapon or Glass Cannon. Side grades would be stuff that lower cooldowns or procs extra hits.
Can't say for sure since the caps work weirdly but it looks like you'd get more room maxing crit and dropping Charged Attack(doesn't work on skills) and the Attack+(unless the + is a good skill). Helps if you roll crit and normal attack cap on overmastery though which can take a while if you're unlucky.
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I started grinding the last boss past few days, rotating a few chars I like there, it's not too hard, at first I had issues soloing it on some chars cause my NPCs were worthless(literally 3 of them couldn't kill their own side before I was done with mine) but once I started getting dmg cap V on them it went a lot smoother, and I got Eugen's weapon as a drop too so switched him in and he does ridiculous damage even as AI(also when I play him I ran him a few times it's pretty fun, does ridiculous damage at range). Now I can do it with fairly weak mains to check them out while doing something useful since my NPCs carry me, but I don't like too many chars in the end.
Charlotta is neat but she lacks damage(could really use another dmg skill) although the invuln trivializes a lot of random fights, Eugen is stupid strong but get a bit boring, Vaseraga is fun but feels meh on Bahamut cause the dmg windows are too short and you can't flex the stupid damage as much, hilarious on normal fights where you just pop the mobs for like 5-10% on charge attacks though, Percival is cool, simple but pretty strong and flashy, Lancelot is like Charlotta but way stronger for no real reason, Ferry I tried again in lategame and while I kinda like her playstyle her dmg is atrocious(pretty sure in theory it's ok if you can get max range dmg but the distance check is ridiculously precise, one step too far you're out of range one step too close you lose 30% dmg, shit's dumb).
I want to try Zeta but I need to buy her dalia sigil which feels bad, same for Io can really only play her with the dalia sigil. Siegfried looks like it was designed without Proud damage in mind you can just combo through hits but you just die if you do that on Proud and if you dodge the timings get all fucky so I haven't bothered gearing him.
Overall I mostly rotate Charlotta, Percival and Eugen. Should probably check Rosetta and Cagliostro again now that I have endgame sigils for cooldowns/charge.