INNO has done a good job of designing Events such that If you plan and play well, you should be able to secure the main Event prize without spending any Diamonds, but there is always the spectre of a bad run of luck.
Don't hesitate to ask for help here in the forum or to expert players in your Guild.
Read the Questline and Event mechanic spoilers (available in the FoE Wikia) and plan accordingly. Keep the spoilers up in a separate window so you can see the upcoming Quests.
Review all the stuff in your Inventory. Coin and Supply Rushes, Coin and Supply Boosts, Buildings, since you never quite know what can help you get through Quests.
Time your city, Aid, Tavern, Tavern sitting, and the Guild Expedition, to be ready for collection just after the Event starts. Be cautious when collecting your city and Tavern and going through GE while in the Rush Questlines. Hold off Aiding and sitting in Taverns until a Quest asks you to do so. Your hood can be a valuable resource for Aid as well as easy battles.
Have a good stock of Coins, Supplies, Goods, and Forge Point Packs in your Inventory.
Pay close attention to the Quest. Does it ask for something from your Era or just something? (example, 'Donate 70 Goods of your Era' versus 'Donate 70 Goods'. Others include Troops, Buildings/Decorations, or do X amount of productions.)
Store a Terrace Farm (if you have one) or otherwise free up some space in the city to be able to complete build Quests.
Blacksmiths (Production Buildings of the Bronze Age) are your friend, so make sure to build a bunch before the Event. Hunters and Spearfigters build and train really fast and are cheap.
If you have many Production Buildings of your age already, you won't need as many Blacksmiths. This will help not waste boosts from the Lighthouse of Alexandria or the Royal Albert Hall.
Recurring Quests can help you with Quests that require you to complete Quests.
Build / Have X amount of Buildings - You do not have to wait for the Buildings to complete, but place them down for the Quest to complete. Don't sell those immediately though, since you may want them for Quests that require you Collect Supplies/Coins.
Gain X amount population - Tear down Barracks or Goods Buildings. They provide a huge amount of Population. If you are rich enough, use a Storage Kit on a large Event Building you already have and place it down shortly afterwards, while making sure it stays in sync with the rest of your daily collections.
Gain X amount of Happiness - Faces of the Ancient, Gates of the Sun God (both available in the Guild Expedition), and cheap Decorations. If you are rich enough, use a One-Up Kit or a Renovation Kit to upgrade an Event Cultural Building to get more Happiness that way.
Collect X amount Supplies or Coins - Build and sell mundane buildings from your Inventory or the Build Menu. Don't wait for them to complete, but rather, sell them as soon as they are planted.
