This is a collection of tips rather than a comprehensive guide. The topic is rather sparse here and on the forums so I'd like to share some common wisdom about buying and spending favor points, for the player with a limited budget who will never spend two nickels on the game.
Free Favor Points
Crucial to the spendthrift is the acquisition of free favor points. If you ever come across an opportunity to procure freebies such as the gifting promos (100 eggs for 30 fp) always put in the work to get maximum profit. That means building a large army who can give you those 100 gifts you need to exchange for favor points.
Limit Your Buys
Luckily, at creation your character gets 20 free fp, and you should buy an oracle general. In fact, you will never waste fp on anything else but oracle generals, refills, and possibly an item needed for a general once in a blue moon or a name change. NEVER gamble on chests, ever, unless you also wish to open your wallet. Never use fp for reinforcement items, gold, or extra army members as these can be had with elbow grease. Be frugal with refills as well. NEVER use them to level up or to save monsters, but limit refills to gold medal attempts, where levelling is simply a free refill for both energy and stam. Refill if you have to complete an excavation quest, or elsewise have anj emergency need for extra stamina or energy in a limited time.
Foresee the Build You Need in the Future
I find that much of my enjoyment becomes limited by the paint-drying boredom of waiting to level up. This limits my build options away from PvP, but if you have the patience to do guild battles you might choose a PvP build, not spend money, and still enjoy the game by gearing your play to staying at a low level, praying for sp, and becoming a great guild battler in regular, festival, or arena guild battles. Dueling and invading, champion and festival duelling gives you lots of play options for a PvP thriftspender, as long as you keep in mind that it will take you 10 years to reach level 600 because without spending fp to level up because you can't farm to get free fp and you won't spend two nickels out of your coin purse to BUY favor, you won't ever level up quickly. In this case you have to abandon getting enough invasion equipment to invade your peers or spend some scarce stam fighting monsters to get your loot. Monster hunting is not easy with low energy and stam, though you might stick to ragnaroks and kessarans purely for their stamina to loot ratios and still get decent invasion stuff. To get duelling items your recourse is arenas, but the really good loot requires monster hunting that PvPers will find tedious and difficult. For example, you need 30 fenixes at 10 million damage (1500 stam) each to make an aegis of ashes. 45k stamina is likely 6 months worth for a PvPer. Out of reach to get BIS? No money, no BIS!
Power Leveling Without Bux
The true route to happiness without spending really is the Monster Hunter->Hybrid build, where you will have a BSI of around 2 and a LSI of 6 or higher. That is similar to PL build and similar strategies apply. Early on, you will want to level up fast and build your energy/stam fast. Then as you get a base of 1000 ENE and 500 STA, you can dedicate yourself to going from 0 BSI to 2 BSI, with an ATT/DEF ratio of 3:1 until you get 1000 ATT/300 DEF. Why? Plenty of activity required to do that, as you power level early, not needing fp to keep your levelling speed maximal. You won't get bored. You will also succeed at early levels in getting PvP ranks to about 10-12 and War rank to 7 by the time you hit level 200. The monsters you kill give good loot for lower damages, and you can quest a lot and accelerate your sp acquisition. At the end of stage 1 and throughout stage 2 (building BSI) you can continue to quest and monster hunt while your levelling speed is still fast even past the gauntlet (lvl 300). Once you get to the end of stage 2, ENE 1000 STA 500 ATT 1000 DEF 100-300, you should be under level 400 and a veteran to the ways of Castle Age. No money spent yet, and 3-6 oracle generals, plus some very good loot to match any level 400. Exsanguinator, Sword of Redemption, Moonfall Amulet, Helm of Dragon Power, Deathwalker Boots, Tentacle Armlet, Swarm of Darkness, Glorious Plate, and decent WAR gear as well. Most of the early gold medals up to possibly Azriel is attainable with 500 stam/1000 attack. But no divine stuff. That's stage 3 and beyond!
Late stage 2 and beyond you should turn to farming to gather favor points. If you are successful you will be able to buy an oracle general fater than the come out and so acquire all the essential oracle generals. No credit card needed, ever. The key is to join fp farming groups, ask the members how to do it, and then dedicate 25%-50% of your stamina to killing skaars, ragnaroks, cronuses, krakens, and genesises.
Hunting Divines, Stage 3
At the same time with the other 50%-75% of your stamina you should hunt divine items or BIS items that come with alchemy. Figure out the total stamina required for a divine item and choose your progression wisely. The three commonly accepted routes are a) jahanna-aurora-agamemnon/ambrosia b) giants-jahanna-agamemnon or c) helm of the deep-living death. Once you get 300 divine power you can hunt anything else. Vermilion is necessary, as is Poseidon, for BIS/divine gloves and magic. At the end of stage 3 you will have 300 divine power, 1000-2000 energy, 500-1000 stam, 1000-1200 attack, 100-500 defense, 150-300 health, level 500. Your slower levelling speed is made up for by increased activity on all fronts, from farming to monster hunting to PvP, and collecting gold medals and achievements will give you many goals to reach for. You will need to occasionally refill during stage 3 as well to get gold medals on the divine monsters.
Stage 4 and beyond
No money after level 500 means slow level speed, UNLESS you keep increasing ENE/STA. That's why no money means monster hunting build almost exclusively. But you will never reach level 1500 so keep that in mind. The game will still have many challenges after level 500, never fear, because the game will continue to spit out new content faster than you can keep up., That's the beauty of paying no money and enjoying castle age!