The first thing is to realise that this is an event to incentivise players to spend money - so if you generally spend money on this game, doing so during the event will get you more bang for your buck. If you don’t spend money, the top rewards will be out of reach, but there are some options to earn gems without spending money:
- Check the offer wall - I’m not sure if they count for the event (I’ve never even had offers in the offer wall) but if you have decent offers you can give it a shot
- If you have unclaimed first-time-clear rewards for Castle floors 30 and above, and you are able to clear it, now’s the time to do it
- Grind campaign worlds 9, 10, or 11 - all of them have 2 hidden treasures that are accessible without perks, and each of them has a 7% chance of gems
- If you have Jackbot unlocked and he’s strong enough to farm at least World 9, you can use him - however, you’d be looking at 3-5 gems every 6. runs or so, so this is not going to yield much. However, the extra energy will be useful and also you could be getting other loot for your trouble.
- Take Treasure Hunter as your highest priority perk (it should anyway be the priority in a farming run), and Fast Learner is the 2nd (to give you more shots at Treasure hunter). Except when you are playing Daily events / Weapon dungeons and doing so would put you at risk of failing the run
- The mystery crates can also throw up gems, but the chances are much lower - so I wouldn’t bother unless the boost helps you progress further in the world you are playing