General answers: Go ahead and open some other kingdoms with gold and do their challenges for souls to level your team. Optionally purchase some keys with gold and see if you get some better cards to fill your lineup with. It might just be that your current team concept is underleveled and thus not working like you want, or that it plays too much to that encounters strengths. Leveling up cards or trying some new ones can help in those cases.
Specific suggestions: I believe the final boss is Sheggra, I don’t remember what her support troops are off the top of my head, so I will stick to suggestions around stopping her. Sheggra is a very powerful legendary especially when you are just starting out.
A good way to hamstring her is to keep the red off the boad as much as possbile. Look into transmuters who can turn the red into something else. Or pack in a troop who powers their attack off red keeping it clear. That way when she triggers there won’t be as much red for her to mix her red creation with and thus there won’t be as many skulls when she transmutes it. It also will help to slow down her charging.
Another option under this general strategy would be cards who drain mana. I don’t want to list a litany of cards you probably don’t have in the legendary and epic tiers (or even ultra rares), but there are a couple in the rare tier you might have. Cockatrice is a good example (plus it adds entangle which is a great combo for fighting sheggra).
That brings me to another great strategy for any boss (or player team) who depends on skulls for damage. Entangle reduces a mobs attack to zero and keeps it from gaining further attack. It wears off like most ailments with a 10% cumulative chance each turn. Once done the mobs attack is restored to the pre-entangle level (basically attack buffs while entangled bounce providing no buff). Keep in mind 4 and 5 of a kind matches still deal 1 damage, but that is so much better than 3 or 4 times the top enemy’s attack rarting! Cast it just before you think Sheggra is about to proc for best effect. It might still wear off the turn after you cast it, so it might take a few tries, but the strat is sound.
Soaking the damage
This one is a dangerous strategy you probably will want to mix with one of the others if things turn against you. You can kind of trick the AI by letting sheggra power up (or even forcing her to power up) just as your first mob is about to die. By about to die I mean one 3 skull match away from death. You can do this by leaving 4 of a kind reds on the board so you know the computer will take it, or just stop denying red once she is close to charging and your mob is close to death. Since her damage doesn’t trample through she will pump a ridiculous amount of damage into the poor first troop on your side. The hope is that there aren’t enough skulls left after the initial match to cause you a problem with subsquent matches. Meanwhile buff your second mob instead of trying to save the first one. The situation has to be just right for it to work well, but nothing more satisfying that watching her dump a devastating attack into a 5 hp mob knowing that attack would have outright killed your next one. Keep in mind, when it backfires (generally by a random drop in skull set killing the first mob prematurely) you lose both your sacrificial mob and the one behind it usually. That rarely ends well which is why I say it is a last resort type strategy.
I hope that helps you out. Others will probably stop by with further suggestions as well. Best of luck!