It’s about having a good variety of heroes for each situation. Last event, Grimmforest had red reflect and a red mobs at each stage so blue heroes are very essential. Ulmer, Valen and Karil are good choices here, some essential heroes are like Wukong, Brienne, Boldtusk/Kiril, defense debuffers. It’s important to have a big roster or even multiples of a good hero. AoE heroes also have their places.
The events also consume A LOT of resources. Mainly Arrows, Axes, Bombs, Dragon Attacks, Mana Potions (small/medium/large), Tornadoes, and Time Stops. The basic premise is to go for speed and big combos/cascades. You can do this by either setting off a good board in the first wave allowing your heroes to gain full mana, nuking the 2nd wave with direct damage items (arrows, axes, bombs, dragon attacks) and then using your hero specials on bosses as well as your leftover items or even using mana to hit them again. You can do this in any order actually as long as you get off some combos/cascades.
You CAN NOT just nuke all 3 waves with mana pots and direct damage items. All of this while trying to get as fast a speed as possible, good rule of thumb is less than 1 minute is pretty great. Then the more health your heroes have at the end the higher the health bonus too. Troops help but they are not the be all end all.