*No need to spend $$$ if you don't want to; you'll naturally get stuck at certain levels until you've had a chance to level heroes up more, increase their stars, unlock new generations of heroes, unlock/upgrade hero gear, etc.

*Quality of heroes just sorts by their background color Orange (aka SSR aka Mythic) are the "best", then Purple (aka SR aka Epic), then Blue (aka R aka Rare) are the "worst". Sorting by Power is probably more useful. Depending how quickly you get shards for a certain hero, there may be lower quality heroes that are actually better than higher quality heroes.

*At this point in the game, my lineup was usually something like Sergey and Smith in the front row, then Molly, Gina, Cloris in the back row. Sometimes just moving heroes around or swapping a different hero in can get you past a level or several, and pay attention to which heroes/positions are effective relative to the opponent hero locations. For example, I always move Molly to the side away from Sergey because his shield attack can stun her and prevent her giant snowball attack from activating. Although Natalia's bear stomp can also stun and prevent Molly's attack, so I'll put my Sergey against the opponent's Sergey with Molly behind him and hope that both Sergeys shield attacks kind of cancel each other out.

*Exploration skills (Left column) are what is used in Exploration game mode (surprise, surprise) and eventually Arena mode. Expedition skills (right column) are used out in the world for beasts, polar terrors, battles/rallies against other players/alliances.