Probably Flower Knight Girl and Millennium War Aigis.

In FKG you just need to level whatever you have and pray to RNG that you clear a map with its random tiles.

MWA does not necessarily need the premium units and you will get currency over time that you can use. However, you still need to know your stuff to clear higher tier maps.

Do note that neither of the game offers a pvp element and that any game that does so will ultimately be a payment trap if you want to be best. FKG and MWA do not really require you to "be the best" and payers can just show off their "awesome" units. However, the maps themselves are dependant on your own knowledge with the units you have.

And chillin, everytime i see your pay to fail comment i feel like punching a kitten.