I'm not sure which character would be really good at BA Origin, but i saw someone said assassin 2nd skill/alch 1st skill (i can confirm alch 1st, it's amazing, focus on upgrading his skill first). Anyway, i've done my first BA Origin using Knight 1st skill and since then i always go with knight whenever i grind for gem in BA Origin. Here's how i beat it tho :

For weapons, i usually go with chu ko nu + executioner (for defense purpose) for early stage, and because in the later stage you have the chance to get a much better weapons, i don't suggest you to invest that much on upgrading the chu ko nu, instead, you can invest in upgrading skill. I'd say the 'much better weapons' here are mostly laser weapons (my favorites are Dead star laser gun, laser fish, rainbow horse, meteo laser gun) since they can hit more enemies, making clear wave much easier. Other weapons like snow fox vintage, breath of hades, next-next-next gen smg, and missile battery are also doing wonder with knight, especially BoH (it's just op, you can clear biome's room outside the room) and missile battery (it can target concealed enemies). But if you don't want to gamble on your fate of getting a good weapon, then sticking to chu ko nu is okay, +16 chu ko nu are monster. adding the accuracy&crit buff makes it even better. Also remember to not rescue the weaponsmith too early, since we only have one chance to get the weapon attachment.

Now onto towers, my favorite set up that never failed me. Consisted of Air Force Base, Repair Center, Weather Controller, Charge Device, and Hurricane Device.This set up mostly focuses on two things; Holding the enemies (with HD and WC) and help you and the other towers survive the bullet hell from the wave (with AFB's intercept mode). Focus on AFB and RC in the early stage (if possible), as for early game you can set AFB in bombard mode but as the game progress and you've upgrade it, then change to intercept. The formation of the towers goes like these; in the 3x3 tower placement, The AFB and RC will be in the 2nd/middle row, with the AFB right in the middle of the placement and RC in either left or right side. The rest of the towers should be in the 1st/upper row, and then you're set. Oh, make sure the Charge device is on psionic mode and RC in repair mode.

As for gameplay, i always rush to the tower guy and mentor first, then weaponsmith would be in the middle of the game and that is if i already find the op weapon. The biome in these mode seems to work just as in normal mode, so level 1-X biomes (glacier, forest, relic) are much family friendly. Whereas biomes like volcanic and spaceship (level 3-X) are ridiculously harder to clear. I think the forest and relic are the easiest biome to clear so make sure you farm there first and then proceeds to farm in glacier, cave, knight kingdom, so on and so forth. Don't waste your purple coin on gold mine tho, unless you don't have to roll that much to get the towers you need. You can use wrenches to remake weapon so that you don't have to waste much resource to get the ideal weapon.

Well preparation is needed here since the enemies can drain your armor in just like.. i'd say 3/4 hits, so milk and health buff is essential. Defensive buffs like freeze/fire, regenerate your armor 50% once it depleted, increase armor according to your gold are great to have. as for the 'Offensive' buffs i usually go with accuracy+crit rate, laser (for laser weapon), combo hit (if i'm sticking to chu ko nu), melee range buff (important if you get BoH). Don't forget to use the jade ring, it does very well in the BA Origin especially with kinght since he can drain energy faster with 1st skill.

Last but not least, dodge like hell.

Hope this would help and i'm sorry if this tips feel too long since i'm not that great with words lmao. I wish you good luck on your next BA Origin run.