First of all, let me join others in pointing out what sweet ASCII art you've done. This definitely has the beginnings of a much nicer text game than most.

I'd start by defining some structures to hold data about specific things in the game. For a couple of examples:

struct Ability {
    std::string name;
    int level_adder;

    int cost(int level, int inte) { 
        return (level + level_adder) * inte;
    }

    bool can_afford(int level, int inte, int manna) { 
        return cost(level, inte) <= manna;
    }

    void show(int level, int inte) { 
        std::cout << name << "[" << cost() << " manna]\n";
    }       
};

class PlayerClass {
    std::string name;
    std::vector<Ability> abilities;
    size_t ability_count() { return abilities.size(); }

    void show(int level, int inte) {
        for (int i=0; i<abilities.size(); i++)
            std::cout << "[" << i << "] ";
            abilities[i].show(level, inte);
    }

    Ability const &operator[](size_t index) const { 
        return abilities.at(index);
    }
};

With these, we can define all the data for the Player classes something like this:

PlayerClass Champion{
    "Champion",
    { "Cleaving Strike", 0},
    { "Melting Thrust", 0},
    {"Critical Bash", 0},
    {"Purify", 1}
};

PlayerClass Necromancer{
    "Necromancer",
    { "Shadow Strike", 0},
    { "cripple", 0},
    { "Mutilate", 0},
    { "Life Tap", 2}
};

...and so on for the other player classes. For only one example, this makes it much easier to add more player classes in the future--for example, I can sit down and decide I want to add a "thief" class:

PlayerClass Thief { 
    "Thief",
    { "Pick Pocket", 0},
    { "Grab Purse", 0},
    { "Rob Business", 1},
    { "Rob Bank", 4}
};

...and most of the rest of the game can work with this new player class without any modification at all.

Then we can define a player to hold (for example) a reference to a PlayerClass object:

class Player { 
    PlayerClass &pclass;
    // ...
};

With this, getability obviously returns (possibly a pointer or reference to) an Ability object, and looks something like this:

player.pclass.show();
cin >> input;
if (input > player.pclass.ability_count())
    ability = None;
else { 
    ability = player.pclass[input];
    player.manna -= ability.cost();
}

Note how this has eliminated huge amounts of repetition in the code, with essentially identical logic repeated once for every ability of every player class.