Focus your early gold on upgrading your base's production rate. In the "Early Warfare" phase, out-producing your opponent is usually more effective than buying expensive single units.

If you are looking to progress faster in the game without compromising your device's security, here are the most effective :

Use cheaper units to draw fire while your high-damage units (like archers or casters) deal damage from the backline.

If the game offers "Double Gold" or "Free Chests" for watching short clips, take them. It’s the fastest way to bridge the gap between levels without spending real money.