It is pretty much impossible to get the mythics you have missed at this point. It takes a month worth of maxed guild tasks with all gems funneled into VIP chests to even have a decent chance at getting a mythic. However, unless you really want one of the ones in the random drop pool, spending resources that can get you a mythic when there is not an exclusive gives you a chance at getting one you already have, which, in most cases, is basically useless. If you had, say, 5 of 9 of the currently released mythics, your average cost in gems/keys for getting a unique mythic from the fully random pool is 2.25 times as high. For each one you miss, this cost moves closer to what it would be during the exclusive period. But each one you pull randomly lowers your chance of getting the future ones. If you had, say, 6 of 9, then the remaining mythics have an average cost of three times normal. And if you were on your last one currently would have an average cost nine times normal, since probability has no memory.
The current best (only) strategy to get every Mythic is to get each of them when they first come out. There is no other reasonable way to get them all, even supplementing with purchases, because those are always random. With a $50 per month purchase and full weekly guild tasks, you would eventually be able to get every mythic, but this could happen months or years down the line depending on what you have now and how lucky you get. Otherwise, if you missed one, get used to not having it ever. So long as you only get enough resources to get (maybe) one a month, on the collection side, it is best to at least guarantee it is one you don't have yet. The alternative is to be in a guild with enough legendary tasks to get an average of an additional mythic per month.
I spent 200 VIP keys (9k gems), 253 gem keys, and 1000 glory keys and did not get this mythic. I may or may not try 50 more VIP, but I need to make sure I have enough reserves for the new kingdom and upcoming legendaries. Getting every other troop is far far more important that mediocre mythics. At this point, most mythics are nothing more than little toys and aren't actually required to progress your kingdom stars. If mythics are ever required for kingdom stars under the current drop scheme, a lot of people are going to be less than pleased.
Event keys have been stated to have the same drop rate for mythics as Gem Keys, and only drop mythics if the current event kingdom has a mythic. If you get to the point where you ever have to spend raw gems on event keys (either to hunt the mythic, or because you spent too much and need them for the next new legendary), keep in mind this is three times as expensive as getting them during their exclusive period with VIP keys. Also keep in mind that estimated drop rate for mythics is probably somewhere between 1/500 and 1/1000 for Gem Keys. I would definitely stop spending event keys in any kingdom you already have all the legendaries ascended to mythics, because at that point, you can't get anything else of value, and even dual legendary kingdoms, this should happen before 500 event keys spent if you started with zero of both.