Lifemiles is bringing back the 100% bonus promotion. Avianca's award chart is here.
Miles from Avianca are the cheapest way to use Star Alliance on award routes currently.
While it may be a good idea to just purchase miles speculatively it's better to do that with a specific flight in mind. Aviancakeeps running these promotions at least once a quarter though you can't be sure there will be a next time.
Log in to your account and search the flights you have in mind and see if there is going to be availability. Avianca is unique that it almost exclusively trusts the computer with possible routes. You can't just make up routes and call without fluent Spanish and tons of patience. The online booking system works well but comes up with odd routes sometimes and only works in one class (i.e. for all the legs between origin and destination the desired class of service must be available or the booking system won't show it.).
At LifeMiles we give you more because we want you to live your miles to the fullest, therefore, LifeMiles brings you miles 2×1 until August 30th. For each package of 1,000 miles you buy, LifeMiles will give you a bonus of 100% more that can be redeemed to fly to more than 1,300 destinations on routes operated by Avianca, AeroGal, partner airlines or Star Alliance member airlines.
Need more LifeMiles to achieve your desired redemption? You can purchase them here in blocks of 1,000 LifeMiles for US$30 + tax, per block. When you purchase miles keep in mind:
Maximum miles to buy during a calendar year: LM150,000
Miles must be purchased in multiples of 1,000
The miles purchased do not apply to achieve or maintain Elite status
The amount paid for the miles' purchase is not refundable
The cost per block of 1,000 LifeMiles may vary according to the channel and country of residence
The miles are not endorsable
Miles purchased, once accrued, can be redeemed in accordance with the conditions specified in the LifeMiles' Program Terms and Conditions and the portfolio of products and services available for redemption
Buy 40% of the miles for a trip you think is of interest to you now. Then keep checking availability and buy the rest when booking.
You can purchase miles here if your account has been active at least 10 days before the start of the promotion.