Since both Wombo Combo and Gold Digger have very grindy requirements that also depend heavily on RNG, there is no advice I can give on making your combos. HOWEVER, as you level up and mana generation becomes easier, you will notice that eventually you will always have 100% Mana for special attacks! This is when you can begin the Cornelia Infinite to easily farm these achievements!
To do this, you must simply fight Sister Cornelia. Whenever you get to her bomb set up, you have the power to stun lock her into oblivion with your ability of prolonging attacks! The special thing about Cornelia is that she can ONLY take damage from her own explosion, and quests. If she doesn't get to set up the requirement list, she will never take damage. This is how you can cheese her own ability to actually keep her alive forever! This makes spamming gems, or looking for 15 combos a walk in the park! She won't get in your way, and she'll never be defeated! GET TO IT!

Note 1: Remember that she has to be stun locked BEFORE the list of requirements shows up, it does nothing if you already have one of her lists up.

Note 2: This is also super safe because Cornelia will never defeat you, even if something goes wrong! She'll just send you back to the world map!