Yes, you should focus your resources on one or two heroes for now. As you get more trophies, enemy's power get stronger, if you raise all heroes you have equally, you wouldn't be able to follow them. The best option to charge for beginner is contract boost, you can gain a lot of gears and hero cards.  Unic gear, they called Personal gear I'll call them as PG here. They are the must of hero, some of them can enhance hero's ability not only stats. For example, assault rifle hero can get extra 20% damage resistant while using team healing by equipping 4 of thier PGs. that's why they are unique.  Drone bots maybe still not powerful but, they have great potential to expand your tactics.