Castle Invasion: Throne Out

You upgrade weapons/skills by collecting coins during battles and then selecting the hammer/anvil at the top of the level select screen. The later levels usually give more coins, so it's best to just keep playing straight through the game and upgrading what you can while you go. You can replay previous levels to collect more coins if you are struggling on a current level to upgrade your weapons and hopefully making things easier for you.

There are 2 skill and 3 slingshot upgrades you need to purchase (in addition to the Bow, Spear and Crossbow):

Movement Speed:
Level 1 - 15G
Level 2 - 50G
Level 3 - 100G

Weapon Pile:
Level 1 - 75G
Level 2 - 150G
Level 3 - 250G

Slingshot Stagger Duration:
Level 1 - 250G
Level 2 - 500G
Level 3 - 750G

Slingshot Stones:
Level 1 - 500G
Level 2 - 800G
Level 3 - 1200G

Slingshot Spread:
Level 1 - 750G
Level 2 - 1000G
Level 3 - 1500G

Once you have upgraded all possible things, the trophy will pop.