Looking to start raiding
I tried with LFG for Raid.
But all my party disbanded after 1-2 failure.
Can’t get those Raid masteries unlock …
For one, I gotta point out that Metabattle is not as active as it used to be so some of the builds are out of date or not quite as optimized as they could be. The folks over at qT are currently the go-to folks when it comes to PvE. I mention that specifically because you mentioned you got your Reaper build from MB where qT has an ever so slightly more optimized build (underline slightly). They also have videos on proper rotations.
Where to start? If your guild doesn’t already have a “raiding chapter” you will either need to look for a guild that can offer that, or you can be on the look out in LFG for groups with the title of “training” These particular groups may not necessarily get kills, but the folks running the groups are more concerned about making sure all the new raiders are familiar with that particular boss’s mechanics and have practice fighting the boss. After enough practice you can venture out to other LFG groups or start your own.
Watch videos of kills to learn the mechanics
I also recommend getting into a guild that has a decent raiding community.
In addition, you should work towards gearing out more characters than just your necro. You don’t need to have full ascended armor, just weapons and trinkets is fine. Being able to play multiple characters is extremely desirable, especially when you get to a boss where your main class is particularly weak. If you make power classes they can all share ascended trinkets and some weapons (e.g. ele, thief, rev all use staff) so you will mostly just need to make the exotic armor, which is pretty cheap. You will be able to build up to ascended armor on all your characters with raid and fotm daily rewards.