Once you are level 30, refrain from levelling as long as all following upgrades are not over!
Increase all buildings to the maximum (your current level). Upgrade Town Hall first, of course. Upgrade Warehouse and Cottage last. Allways Upgrade Academy enough to allow you to keep upgrading Technologies.
Upgrade all useful Technologies to the maximum. Useless Technologies are Troop MATK (unless you wish to use Angels or Warlocks) and Hero PATK for mages or Hero MATK for other classes.
When Academy will reach level 41, you will be able to use Blessing Wheel. Try to use it each day until you get a +200% boost to Academy bonuses, expect to spend around 20,000 Kyanites per day if you are a bit unlucky.
Upgrade your current best brawler creature to the maximum with daru, it is worth it, but skip creature as soon as the next one is available.
Some players prefer magical creature because they can heal. It you think they are worth it, increase Troop MATK instead of Troop PATK of course. But they are no good magical creature for level 30-39, so you will have to use Gryphons.
Those are two extremely expensive ways (in gold) to improve your character. To avoid losing gold by being plunder, if you have no magic item to enchant, allways use one of these ways to spend you gold!
To make things more clear, let's call the primary ability improving your attack the major (Intellect for Mages, Power for Knights and Archers) and let's call the one incresing the attack you don't use the minor.
Contribute to your guild, allways by multiples of 1000 Gold. Use contribution to buy Guild Skills. Gold Skill is cheap, so think in long term and upgrade it first.
Then upgrade everything but minor Guild Skill as those are somewhat cheap. Take even first level of minor Guild Skill as this also increases your weakest defense.
Next, you should mainly focus on your major Guild Skill.
Try to get Astrals with the remaining money. Do not keep green Astrals, exept if you have nothing more to wear. It's better to sell them because they give too weak Astral experience. So set the listbox on right with "Green and below". Use the "One-click Sell" button then the "One-click Collect" one once you have captured some Astrals.
Don't forget that when you use an Astral to energize another one, you get back this way all the Astral experience he has earned. So a mage could use a green MATK Astral until he get a blue one, energize a blue Astral MATK once he got one using the first one, and so forth with a pink MATK one then a yellow one, then a red one.
Recommended Astrals for mages are MATK, PDEF, MDEF, HPs, Will destroyer, Goddess Blessing. Recommended Astrals for knights are PATK, MDEF, HPs, Block, PDEF, Goddess Blessing. Recommended Astrals for archers are PATK, Critical, Sniper's Edge, Goddess Blessing, MDEF, PDEF, HPs. Those are not the only ones to play. Allways keep an astral if it could be interresting latter, when you will be able to wear more. And don't forget it's easier and less expensive to change equiped Astrals than to change Gems in equipment for instance. HPs Astrals are useful only when your HPs are still low.
Save Astral Points for red Astrals only. You should not buy a red Astral this way before you can upgrade it to level 5. This mean you get the matching yellow Astral to level 5 with 13,700 experience more, or a pink Astral to level 6 and 14,700 experience more.
Each day, the first Astral you capture is free. So you can consider to try to end the day with a high active star.
This is another thing which requires a lot of gold.
Identifying an Helm gives Intellect, an Armor gives Defense, a Weapon gives Strengh and Wings give Endurance. If you have no Fashion Core for Refinement, the most profitable is to try indentifying your D quality Clothes (after the matching level 8 Guild Skill), then the C quality ones (after level 9 Guild Skill), next those of quality B, S and then A (all after level 10 Guild Skill).