Been playing the game for about 2.5 years ( joined October 2014 ) , never spent money in this game. Here is my mech. Working on getting a few more cool down modules. I had a yoshimo power level 12, but that somehow glitched out and got deleted from the game.
Just play the game for 2 or more years and you will get many good items
Also wait for sales on gold boxes. Never use gold boxes if there is no sale, because then chance of mythicals drop significantly. I recommend only using your free gold boxes on July 4th and Black Friday, on those two days the best sales of the year take place. Also do the hard and insane missions, you get a decent amount of tokens for doing all of them. Prioritize cooling and regeneration the most. In rank one, try having more than 130 cooling and more than 50 regen. It prevents most heat mechs from double cooling you, and prevents energy mechs from dealing massive extra damage to you ( instead of dealing 150 Damage per turn, only one of the turns will deal massive damage). For HP, try keeping it above 700 if you don't use godmode, and over 1100 if you do use godmode. If you have that much HP, you won't need to worry about the negative resistances until late game. I used these "rules", and it got me to all stars on rank one. If no godmode, consider using the +10 multi resistance or +11 multi resistance, since it is like extra HP. If you have any extra items that are not in use, consider using them into your torso. Your torso should be highly fused, that gives a lot of extra HP. Fusing into a weapon only adds one damage, and same for legs. You won't really notice a difference in fused weapons until they are at least level 10, that is where the extra damage is enough to be helpful. Watch videos to get tokens, farm for credits to get more items for fusing.