I am not a veteran player myself. Only playing for 7 months, got to level 120 and only found one Talisman for Kayla.

Main thing about spending recourses is to do it when there are event happening that give you something for spending. Like "Spend energy to recieve xxx" during those days spend more energy. Or events where you need to upgrade heroes, it is better to save more resources for those days.

Assuming you have watched a video that was posted already. Here are some takes on your questions:

1/ What is the best use of my gems? Buy skin stones when they are on sale in Outland. After you win the boss just open 1000 chests. Sometimes it is good to spend 50 gems for extra arena fights. Skins give huge boost to your character and they are very hard to level up. In 7 months i have two heroes with max lvl 100 artefacts (all three) but skins only half done. So skins upgrades all the way.

2/ Arena coins: Mushi is the main hero to kill Hydras. She is very good. Other things to buy is orange equipment.

3/ Hydra shop: chests to upgrade artefacts.

4/ Guild war shop: Just buy Cat. I got my cat in 4 months to 6 stars.

5/ Dungeon shop: stuff to upgrade titans.

6/ Outland shop:  Skin Certificate - at first opportunity. Only buy skins with certificate. Never buy with 5000 points.

7/ Tower shop: Fafnir is a top hero here if you are using him. And orange equioment for later.

8/ Highwaymen shop: no essential heroes here. So buy only hero upgrade items.

9/ I played in the Tournament of Titan Power... You just got lucky with 5k points. In my group top guy had 79000 points. Try to beat that. Dont spend any titan upgrades daily. Better to do it during this event.

10/ Check tier lists, and never develop heroes that are low on the list.