Sadly, it is with real money using a credit card and buying gold, tokens and premium packs. If you ask for free to play, then the answer is grinding every day to get mix boxes and gold as basic it is sounds.
How many times to farm a day to progress fast? The basic answer will be 4 times a day to get average 350-400k plus collecting gold from base that add 100k and 5 wins for a close 100k depending on ranks. So, if farm 4 times let's say 400+100+100 you can earn around 600k per day with just 5 wins. Let say for you been a new player, lets round 400k.
With the 4 times farm full cap, just for saying, depending on your campaign choice place and difficulty, that mean having 60 in fuel cap in OD6 Insane, can farm 8 times per full cap making that a total of 32+ grinds having a possibility of 50% to get a box making that around 16 mix boxes to collect a day or so depending of choices, can farm the options of less fuel and same rate of grants and can get more mix boxes and advance faster. In a week you can push hard the level of your mechs without the BASE.
If decide to use BASE, depending on your acc status, can be costly and you need to ensure you are constant in the grinding per day average. Because you will earn less for now at your acc and rank status, it might require also extra PVP to get some extra gold.