I think martingale strategy is good because it is not gambling anyway so you don't totally lose your money in case the price went down but it would just lose value. Buying those coins that have been existing for a long time s a good strategy because you know that there are demand for those coins as oppose to the new coins that has the tendency to lose demand because the investors nowadays are losing their confidence when it comes to cryptocurrency.
If you want too invest for the long term then choose those coins that has good fundamentals and if possible those coins that are use for its actual use case because you know that there are demand for those. There may be coins that are cheap for now but if those coins are abandoned by the team or there are no progress at all from those project then better look for other coins that has the potential.