A lot of folks have been complaining about some bosses being too powerful in Missions Architect arcs. Players not being able to handle a boss generally does not impress the player, it annoys and maybe even enrages. These players may either not complete your arc, or give you a low rating which no architect wants.
I personally have several arcs with tough bosses in them. Here are a few tricks that I've found useful in creating tough, interesting, and manageable bosses...
1 - Take your time in developing your bosses. Think their concept through thoroughly and make the boss' powers fit your concept, but choose the powersets very carefully with special attention paid to secondary effects. You may like the way electricity looks, but no one likes being without endurance, for instance. Definitely go further than "OMG HE's SO POWERFUL NO ONE CAN BEAT HIM!!!!!!!!"
2 - If your concept of your boss is for it to be very powerful, make fighting it optional. That way your players can take it on if they choose to, or work around it. This will make you think about the missions a bit more, but you may be pleasantly surprised with what you come up with when being creative. I've had missions develop in totally different directions than I originally conceived just by having to think around the boss.
3 - If you decide that the boss needs to be very powerful and must be defeated for mission completion, consider either setting them to "Boss" (not EB/AV), and/or limiting their powers to Standard or Hard. I like to focus on 1 powerset over another, so I often set an EB to be Hard primary and Standard secondary, for example. If I want them to have a lot of powers I set them to Boss. I learned this when testing an early arc where I had the players battling a Dark Miasma/Dark Blast EB on Extreme. Doesn't sound too bad, but I found that anything without mez protection can get slaughtered, and some builds with mez protection will have a rough time too. That one taught me a lot...
4 - A trick to making a boss fight more interesting without making the boss too difficult to defeat is to add ambushes. Bosses can trigger an ambush at 1/4, 1/2, and 3/4 health, and also after being defeated. Make the boss strong enough to last as the ambushes happen, but not so tough that players never get to see the ambushes at all because they keep faceplanting.
5 - Think of the poor solo'er, please. Making a Will Power/Fire Blast AV on Extreme means, very simply, that most solo'ers won't be able to do your arc, period.
6 - Limit the number of bosses in your missions. One of the biggest complaints I see on the boards is that a mission has too many EB/AVs. I generally agree with this. Walking into the final room of a map to see multiple AVs can be quite disheartening, especially if the rest of the arc/mission has been hard already. We all acknowledge that multiple AVs is a sight to behold, but not one most people relish.
7 - Be VERY careful about making your bosses flee, especially with a lot of hit points left... on small maps... with placement set to front... ungh. On outdoor maps it can be worse if the boss has fly. I like Fly. I take it on most of my characters. I think flying bosses are kinda cool, too. That said, flying, fleeing bosses stink on ice. Seriously though, bosses escaping can be fine if it moves the story forward, but maddening if it means that you fail the last mission of an arc and won't get a crack at that boss again unless you do the whole thing over. Consider fleeing bosses carefully, especially if you make them very tough.
8 - Test, Test, TEST!!! Do it with different characters at different levels. You have no idea who will be trying your missions. What your IO'ed Tanker can handle solo and what my SO'ed Defender can handle solo will often be 2 very different things. That brings up another point...
9 - Warn the players in the mission description, mission intro text, and mission send off text if a powerful boss will be in the arc/mission. This may be the most important thing you can do because it let's players know what they're getting into. If they feel like a challenge, then they'll play it. Others can avoid wasting time in missions only to get to the last one of the arc to find that they can't finish it.
A Final Note
Not specific to bosses, but in general I say always give other architects feedback, and rate their arcs honestly. I'd hate for someone to tell me "I couldn't finish because the boss was too hard, but I gave you 5 stars anyway for your story". Arcs should be enjoyable, well-written, and playable. Let other people know what they can improve in their arcs (nicely, hehe)... it'll just make them better architects.
9 Ideas for Building a Better Custom Boss

10 is overused.
A lot of folks have been complaining about some bosses being too powerful in Missions Architect arcs. Players not being able to handle a boss generally does not impress the player, it annoys and maybe even enrages. These players may either not complete your arc, or give you a low rating which no architect wants.
I personally have several arcs with tough bosses in them. Here are a few tricks that I've found useful in creating tough, interesting, and manageable bosses...
1 - Take your time in developing your bosses. Think their concept through thoroughly and make the boss' powers fit your concept, but choose the powersets very carefully with special attention paid to secondary effects. You may like the way electricity looks, but no one likes being without endurance, for instance. Definitely go further than "OMG HE's SO POWERFUL NO ONE CAN BEAT HIM!!!!!!!!"
2 - If your concept of your boss is for it to be very powerful, make fighting it optional. That way your players can take it on if they choose to, or work around it. This will make you think about the missions a bit more, but you may be pleasantly surprised with what you come up with when being creative. I've had missions develop in totally different directions than I originally conceived just by having to think around the boss.
3 - If you decide that the boss needs to be very powerful and must be defeated for mission completion, consider either setting them to "Boss" (not EB/AV), and/or limiting their powers to Standard or Hard. I like to focus on 1 powerset over another, so I often set an EB to be Hard primary and Standard secondary, for example. If I want them to have a lot of powers I set them to Boss. I learned this when testing an early arc where I had the players battling a Dark Miasma/Dark Blast EB on Extreme. Doesn't sound too bad, but I found that anything without mez protection can get slaughtered, and some builds with mez protection will have a rough time too. That one taught me a lot...
4 - A trick to making a boss fight more interesting without making the boss too difficult to defeat is to add ambushes. Bosses can trigger an ambush at 1/4, 1/2, and 3/4 health, and also after being defeated. Make the boss strong enough to last as the ambushes happen, but not so tough that players never get to see the ambushes at all because they keep faceplanting.
5 - Think of the poor solo'er, please. Making a Will Power/Fire Blast AV on Extreme means, very simply, that most solo'ers won't be able to do your arc, period.
6 - Limit the number of bosses in your missions. One of the biggest complaints I see on the boards is that a mission has too many EB/AVs. I generally agree with this. Walking into the final room of a map to see multiple AVs can be quite disheartening, especially if the rest of the arc/mission has been hard already. We all acknowledge that multiple AVs is a sight to behold, but not one most people relish.
7 - Be VERY careful about making your bosses flee, especially with a lot of hit points left... on small maps... with placement set to front... ungh. On outdoor maps it can be worse if the boss has fly. I like Fly. I take it on most of my characters. I think flying bosses are kinda cool, too. That said, flying, fleeing bosses stink on ice. Seriously though, bosses escaping can be fine if it moves the story forward, but maddening if it means that you fail the last mission of an arc and won't get a crack at that boss again unless you do the whole thing over. Consider fleeing bosses carefully, especially if you make them very tough.
8 - Test, Test, TEST!!! Do it with different characters at different levels. You have no idea who will be trying your missions. What your IO'ed Tanker can handle solo and what my SO'ed Defender can handle solo will often be 2 very different things. That brings up another point...
9 - Warn the players in the mission description, mission intro text, and mission send off text if a powerful boss will be in the arc/mission. This may be the most important thing you can do because it let's players know what they're getting into. If they feel like a challenge, then they'll play it. Others can avoid wasting time in missions only to get to the last one of the arc to find that they can't finish it.
A Final Note
Not specific to bosses, but in general I say always give other architects feedback, and rate their arcs honestly. I'd hate for someone to tell me "I couldn't finish because the boss was too hard, but I gave you 5 stars anyway for your story". Arcs should be enjoyable, well-written, and playable. Let other people know what they can improve in their arcs (nicely, hehe)... it'll just make them better architects.
Thanks for reading, and apologies for typos