Halo 5: Guardians

For Dangerous game, you must either earn the killing blow or contribute heavily to damage dealt to a Mythic boss in Warzone Firefight. You also likely need to be alive and near it when it dies, no matter how much damage you do. I did over 50% of a Wraith mythic boss's health by myself then died, and didn't get credit when my team mopped him up. Mythic bosses are just like Legendary bosses from vanilla Warzone, except they are stronger and are denoted by red triangles rather than yellow on the HUD. They don't always show up, and can occur on any map, but only if you get the right variant of the objectives.

In my matches to date, I have found that the easiest Mythic boss to get credit for BY FAR is the Wraith bosses. I've seen them in the Attack on Sanctum map and on Raid on Apex 7 so far. On Sanctum, it spawned four mythic Wraiths, but they're much softer than the mythic Knight boss or the mythic Warden. On Apex 7, we had to kill a Legendary Phantom, followed by a single Mythic Wraith with some ghosts supporting it. Only having one Wrait there makes it harder to get credit than having the FOUR mythic Wraiths on Sanctum, especially if you're not in position before the Phantom dies. Apex 7 also shows up much less frequently, at least for me.

Make sure you have a level 6 REQ available and call in a Scorpion tank or a Banshee, depending on your play style, and just unload on one of the Wraiths from as far away as possible. The banshee might be REQ 7, not sure as I dont have any cards for it atm. If in the tank, try to aim your shot for the rear for more damage and a faster/easier kill. If you haven't gotten to REQ 6, a Wraith from REQ 5 should work too.

Another one that was pretty easy to get credit for was the mythic Hunter bosses. I ran across these in March on Stormbreak, in a covenant-wave version of that variant. Tank, Wraith, or Mantis are your best bets for these guys. At the end of the day, as far as I can tell you stand a chance to get just about any of the mythic boss options on any map, just depending on what wave variants you get. Although I kinda doubt you would get the four Wraiths mythic boss on Stormbreak due to the confined nature of the map.

The problem with the mythic Knights and Wardens is that the team has to keep constant fire on them. They spawn Watchers that regenerate their health if you let them hide long enough. They can be a royal PITA, especially since the AI for the last one always seems to try and hide.

The other solutions are already quite clear. But I found myself struggling to get those heavy weapons to unlock all the time, e.g., those level 6 heavy vehicles. For those who are as bad as me at FPS games, here is what you can do:
If your team can make it to the 5th round of a game, there usually are some players skilled enough to get tanks very early. Now, just jump in the tank's second seat and take charge of the machine gun. These players are usually good at driving tanks and will find a good spot to attack those mythic bosses. So just smash your bullets on the boss and usually you will get your credit for the achievement.
I know this does not sound that honorable, but it could be a solution for some people.