This is not as hard as people have been made to think it is. The present mainstream approach of the past few decades is just not a good one. It is picking strategies that are very expensive to implement, and that attack vulnerabilities that are peculiar to tiny subtypes of cancer. There are hundreds of subtypes of cancer, most of which are cheerfully capable of evolving their way around many of the existing attacks on their biochemical vulnerabilities. There are only so many scientists, only so much funding. If it takes twenty man-years to produce a therapy to tackle transient vulnerability A in subtype 1 of 1000, we'll never see the end of it. Yet this is largely what is happening.
What should be happening instead is the whole-hearted pursuit of vulnerabilities that are common to many, many types of cancer, or all cancers for preference.
There is one very good one, which is that cancers must lengthen their telomeres, and there are only two ways for that to happen, neither of which should really be operating in somatic cells. So block telomerase, block ALT, and get that hooked up to some kind of cancer-cell-targeting-mechanism of the sort that has been under development for the past decade, and that will work to kill any cancer. A number of high profile groups are working on telomerase sabotage ( https://www.eurekalert.org/pub_releases/2015-01/usmc-rtt1231... ), and the SENS Research Foundation has been funding the necessary research to address ALT ( http://www.sens.org/research/introduction-to-sens-research/c... ), still at comparatively early stages.
There are other, prospective targets. The comparative biology crowd is reverse engineering naked mole rats to figure out what it is that makes them next to immune to cancer. There are a few leads there; p21, ARF, hyaluronan, alpha2-macroglobulin, etc. Another recent discovery relates to re-engaging the limitation on processing nutrients; apparently all cells have a food limiter, and all cancers abuse that limiter in order to enable rampant replication. Re-enable the limiter and cancers wither: https://www.salk.edu/news-release/salk-scientists-curb-growt...
So there is no shortage of possibilities for people who want to kill all cancer. There is no excuse for continuing the wasteful business as usual of the past decades. This is an age of revolutionary progress in biotechnology - we should act like it and aim high.