I have some suggestions for the multiplayer format that could balance out some of the issues and level the playing field a little bit.
First off I think that attack is a little too easy to upgrade, I think that there should either be a sliding scale where the increases go up in cost depending on their damage range. The current formula is (att-10)10 to advance the attack and once it gets over 30 it is getting a bit crazy. for example miri can get to 34 attack for under 2k Skills while the decision would be cheaper than the other choices of a 3rd spell slot or 4 hp. I think something that would make hp more worth its cost is to double the amount you get for an advancement so you would get 2 hp instead of one each time you buy it and that could make it more tempting to be able to survive those crazy high hit attacks.
a couple ways to make attack more costly to increase to crazy levels would be to either use something like (att-10) * (10+(att-10)) or in between <15 it costs (att-10)10, 15+ (att-10)(10+x) x could be something like (num((att-10)/5) where x is an integer and num is how much you want to increase the ranges by.
I think that another fun implementation that could lead to tournaments or more interesting/ complex clan battles is if you could limit spells like in army mode. So, there would only be one of each type of spell per side, which would limit the first round first turn deaths, cause more different spells to be used. teleport fury would still be good but not as ultimate. the increased damage could be dealt with more by forcing those people to attack a group. more diverse characters would again be more useful, making people that can shoot, lay vines, and move forward more interesting choices. This could be another mode even called hero army mode, I think that limiting spells alone would make this very interesting and have people choose different spells for their builds. also since there are at least decent spells at the beginning everyone could have good characters if there is a little bit of level diversity on a team.