- Relevant to lore and game setting. Not just "Halloween special" or something random. Use the rich Battletech setting to create a cool context. Inner sphere conflicts, Clan invasion, whatever. YES!
- Make the reward something useful that can only be bought for MC. Mechbays, camo patterns, camo colors. YES!
- Make it a skill-based achievement, not based on farming. Sadly, NO.
Let me explain why skill-based achievements are fun, and farming is boring and bad.
- Getting x amount of victories, kills, assists or component destructions in y number of matches requires virtually no skill at all, unless the bar is raised to an obscene level. Anyone can do it, if they invest enough time. That in itself makes it boring for the player, because you realize it's just a matter of time.
- Grinding achievements often result in bad gameplay, because people are willing to do anything to hasten the necessary grind. E.g. risk FF to get 20 kill steals as quickly as possible.
- Getting x amount of kills, component destructions, savior kills, damage, match score or base caps in a single match is much more dependent on skill. There's no easy way to fake 1000 dmg, 100 point match score or 6 kills in the public queue (private matches are different). This makes it fun, because you know you have to play well to do it. It's not just a matter of time.
- Any criteria you set for a challenge can be abused and lead to griefing, but some criteria are better than others.
- Grinding achievements may lead to more people playing, but it's pointless if it becomes a chore anyway. And there's no sense of accomplishment, except that you won a mechbay worth less than 3 dollars. A skill-based challenge gives a sense of accomplishments.