A lot of heroes' soulstones can be either farmed from stages, or bought in the various shops with currency given out daily from ranking/participation. Recently a bunch of older heroes were moved from shops to story stages, mainly the high-tier ch15-ch18, so they're harder to obtain now, sadly. Here's a list of units I suggest looking into (I use them a lot): Note that this is a very long-term focused game, as maxing a character to 7* requires around 1000 soulstones. This takes months.
Zem - crucible - tanker/killer/debuffer, very good moveset and high survivability thanks to stats and self-healing, when ultimate is active deals medium magic damage in a large area every second, excellent vs groups; tombstone acts as bait; awakened gains aoe slowdown
Valan - ch6, 12, 18 - tanker, awakened and wearing armor skin (was possible to get for free a few times) is an unstoppable monster that shields and buffs the whole party. He will carry your party in ch18 and counters nightmare Hurok in prophecy pool. Without armor he'll be in trouble, though.
Baltor - ch15 (formerly garena shop) - strongest non-paid back-line dps, moves deal splash damage when awakened, attacks front, back and mid, easily murders multiple characters with one hit, but is easily stunlocked/interrupted
Sylphi - ch12, 13 (formerly premium hero) - mid-row aoe mage with insane survivability thanks to high stats and ultimate that restores hp to full and charges rapidly, awakened buffs mpierce of all females in party
Taurus - crucible - mage, buffs party, then aoe stuns, then attacks back/mid row for high damage; ultimate is very high aoe nuke with stun; awakened drains ult gauges with each hit; a must-have for all battles, get him to 7* awakened asap
Flora - ch2, 4, 7 - starter hero, healer, very basic and straightforward but can keep your party alive in dire moments; still no awakening; helps me keep my units at full health in crucible; can be trained in hero academy for decent ult gauge recovery and evasion; somewhat tankier than mages, can tank certain enemies; reduces magic damage by 10%
Osiris - ch4, 8, 11 - mage, ST/splash nuker, low survivability, decent energy efficiency, attacks and ultimate deal very high damage (30k+), awakened drains enemy matk and amplifies own for even more damage; boosts magic attack of all party members by 400.
Cara - hall of legends - mage?, causes ailments and medium magic damage, ultimate traps nearest enemy and deals very high damage (guaranteed kill vs non-tanks); initial move is sleep on 2 back row units which buys some time; awakened gains a guaranteed one-time death recovery; a lot of crucible teams use her
and on and on... this game has a lot of unique heroes, and certain battles are most suited to specific combinations of these heroes. For example, the prophecy pool boss fights are very picky and require maxed characters to clear all difficulties.
If you can get your hands on premium Mazir and/or Alecia, do so - a team of Mazir, Alecia, Sylphi, Osiris, Taurus annihilate everything in crucible. Petros was just released in shop and is a monstrous tank. Nighshade and now Vulko are there for back row assassination. Nilya (ice mage) is in almost every crucible party for some reason. So is the ever annoying Adeline (puppet). I've been told Lingling is a great buffer. Kong's (monkey) clones provide good distraction. Octo has heavy life steal, hard to stop during ultimate. Tareth's spear ultimate murders anyone that's not a physical tank, esp. teleporters and assassins. Ethera (alien) is a glass cannon, but can help clear some fights and her ultimate is unique.
The green jade is a super annoying thing to f2p since it discards the jade everytime it ends. Almost all heroes listed can be obtained through normal gameplay, so it has limtied use. I think even the ones at the bottom can be gained with luck from the final crucible chest. My plan is to save up 50 diamond tickets, get some jade from free chests, open a few with diamonds, and get a 600 hero. A more direct strat would be to save up 6x 2600 diamonds but that takes forever. Unfortunately the best picks are probably Alecia and maybe one or two others. Mazir isn't there for some reason. The most abused premium units aren't even in the pool (the vampire and the paper doll, for example).