What's the easiest way to earn or get the 20,000 to continue Sen's route?
Also how do I find Sen at night?
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Crafting and then selling items to Nibbles at his pub after you get so far into his questline.
The best items to craft are elemental weapons, you can scavenge for elemental items in the level 2 area of each 'zone', which opens up after you help out the hunter, Krasz, with his questline.
You can only meet Sen at night after you take a seed of chaos, one is found after defeating the fiend in Russ' first questline series, or randomly at night while scavenging. Once you take a seed of chaos, you have to finish helping him out with rebuilding the town sanctuary.