Good question, Max.
The answer hinges (the verb to hinge means to depend in this case) on whether or not 'to,' is a preposition in this sentence. You can test this by adding the word 'it,' after to and seeing if you can make a sensible sentence if you put a full-stop after the it.
So the question is whether (or not) the sentence 'Ten steps to it,' makes sense.
If you think 'Ten steps to it,' makes sense then you must use the gerund form ('achieving) in this sentence.
In my opinion the above sentence does not make sense so I would say 'to achieve.'
Another test is whether or not you can put 'the,' in front of the word that comes after the to.
The noun form of the verb 'to achieve,' is achievement.
So this would also suggest that we should not use the gerund here.
And yet....I'm sure that the formation '10 steps to achieving greatness,' is used quite a lot.