For competitive key terms you always need authority.
Thus this means, if you don't have a good content / viral strategy, you have to pay for these links monthly xxx-x.xxx$+ depending on your competition / budget.
Aim for domains with a high Page Authority, aswell as Domain & a high trust factor.
Of course, everything else you have to keep natural.
All these link building software you can use for tier2+ as mentioned. It's more about to get a higher domain popularity, but you won't get much trust of it.