To level up a character (bring it up to 5* level 100), you will need a lot of resources. Most players can find their way to 4* level 80 characters easy enough, so we won't get into that too much. However, the last jump to 5* costs 175,000 gold, 9 of the 3* colorless Evo shards and 6 of the 3* colored Evo shards.
We've talked about the gold farming already, so we won't mention that again. XP and EVO shards, however, are a different animal.
You have the choice: cosmos levels take less sands of time, but Lord of Light will give out more XP shards than the cosmos levels. The only place to get the 3* colored evo shards is in LoL, so you will need to farm it plenty.
Same story after the last ressource update : if You can reliably farm Night City, you will get huge amounts of XP shards there, but only for some classes (warrior and assassin).
This is real handy, but can be costly (in gold) if you need to level magus, sentinel or gunner toons.
Farming for XP (not XP shards) is a different technique altogether.
You can use the environment to earn XP for your eddies and allies, the idea there is to repeat an easy level with a strong character, allowing weaker characters in the team to earn XP quickly. For example, you can run powerslave with a maxed out pharaoh/crusader eddie, bringing along two weak (level 1) allies and two weak eddies riding shotgun. This will net you 4K XP per run for each character. This is ideal to level duplicates at a slow pace in preparation for the sacrifice event. You can increase the output by buying either the XP doubler item from the store, or the Sands of time drip pack (so you can do more runs).
This technique works well from the tutorial and you'll use it for a long time, you will only quit when your XP shards income becomes stable.