So the Riker boss mask;
1) doesn't look as good as the others
2) nobody knows if you have it unless you wear it
3) has no function other than vanity
4) is soul crushingly laborious to obtain
The 'need' to get it is nothing more than mob mentality consumerism, like Black Friday. So have a moment of introspection, say no, and walk away smiling because you make good life decisions. When you see someone with it, don't gloat, you were like that once.
it counts one riker boss as two kills so you need to kill 75 - that first of all.
Second of all, Tier 1, Lexington, Sniper build or any other one-hot-high-dmg build (f.e. sticky bomb), She is sittin on a chiar so you just shot her and let yourself die by killed by her guards (+ on a stage is a gas cane which can help you do a general commendation "disable 20 traps using gunfire")
It only counts if you fire the kill shot like other's have stated and yes you can be in a group. You can get this mask really easy(sorry if this stuff has been said after that dude put his little wimp glitch way I quit reading) by doing hvts both weekly and daily the dz bosses open world name bosses underground etc. It's not as hard as the description implies. Just Google rikers and it list all rikers bosses and they all count