How to get base-legendary itmes without coins:
1. Farm the final mission on 1v1 on hard.
2. Arena rewards.
3. Clan war loot.
4. Special offers.
5. Portals.
6. 30-day log in reward.
7. Tokens for single fortune box (not recommended).
8? Opening timer boxes for fortune boxes. Although this only works if you selected it from arena rewards, I included it because you don't need to keep spending coins to keep getting boxes. Up to you to if it counts.
By the way, this entire game takes forever. If you keep playing with the mindset to win it'll get frustrating fast. Getting every item to divine would probably take decades or thousands of dollars. Once you get the shield, there will be another item you need to keep advancing. This cycle is endless.
I currently need divine relics, armor plates, individual resistance fortresses, overload protectors, combined engines, distance shredders, resistance drainers, and seraph blades, just to optimize my current physical mechs. This is after 3 years of grinding and I'd need even more items for different playstyles or mech types. Good luck and take it easy!