There is some good advice and some bad advice here. Let the players play whatever they want. If they want to play all strikers, just design your campaign and your encounters around it. This isn't so hard as people are making it sound. Killing your party so they can start over as a balanced group will just create unhappy players.
How do you make balanced encounters for a Striker party? Let's look at advice from the DMG:
No Defender: This means enemy brutes will become particularly dangerous, and soldiers will have better battlefield control.
No Leader: Healing potions will help. Enemy controllers will become more dangerous.
No Controller: Large groups of monsters and minions become a problem.
Solutions:
Against Soldiers: A Warlock can act as a pseudo battlefield controller with various effects. Especially Star pact warlocks are good at this. Add some temporary hitpoints from various effects, and the Warlock can cover any problems with enemy Soldiers. A Melee Ranger should also be able to go toe to toe with a Soldier. Soldiers don't do too much damage, and what damage they do can be recovered with potions.
Against Brutes: Brutes do a lot of damage, but so do strikers. As long as there isn't more than one Brute in the enemy group, strikers should be able to take down Brutes quickly enough despite their high hitpoints. I would use brutes sparingly, maybe once a day.
Against Controllers: The solution for a group of Strikers is to take down that Controller fast. Otherwise, they may be in a lot of hurt. You should use Controllers less frequently, and when you do, you want to make the Strikers aware who the most dangerous enemy is. Also in most encounter templates, you'll see the Controller is a couple levels higher than the party level. I would reduce that level by one.
Against Minions: Rangers have a lot of abilities that generate multiple attacks. They may not be as good as a controller, but especially if you have two rangers in the group, they can easily cover the abscence of a controller against minions. A dragonborn in the party can also make a difference against minions. Minions don't do a lot of damage, so they are managable as long as you don't go overboard with the number of minions you use in any one encounter.
Against Large Groups: Without a leader or controller, big fights are a definite problem. Easy solution is to avoid them. Give the players a way to hit and run, weakening the enemy. Or give them environmental advantages so they can maybe drop a portcullis to separate the enemy into two groups, and deal with a smaller group at a time. Also make sure the group has some healing potions before any big fight.
So in one day's worth of encounters you can use some Artillery, one Brute, maybe one controller, some lurkers, some minions, and lots of skirmishers. Throw in the occasional Elite, Solo, and Leader, you are really not looking at that big of a difference between a normal party and an all striker party, except the controllers they face will be less potent than controllers a regular party tends to face.
For campaign considerations, like some people have suggested, try to treat them as a special Ops force. Give them more surgical strike missions, hit and run missions, rescue missions, spy missions, etc. A focused party like you have is a rare thing, so you should embrace it and expand the possibilities. Rangers, Warlocks, and Rogues can all be very stealthy, so you can easily have to whole party involved in stealth and scouting missions. They can quietly take out guards. It really gives you a chance for a unique campaign. Don't look at the all striker party as a problem, it's a blessing.
How do you make balanced encounters for a Striker party? Let's look at advice from the DMG:
No Defender: This means enemy brutes will become particularly dangerous, and soldiers will have better battlefield control.
No Leader: Healing potions will help. Enemy controllers will become more dangerous.
No Controller: Large groups of monsters and minions become a problem.
Solutions:
Against Soldiers: A Warlock can act as a pseudo battlefield controller with various effects. Especially Star pact warlocks are good at this. Add some temporary hitpoints from various effects, and the Warlock can cover any problems with enemy Soldiers. A Melee Ranger should also be able to go toe to toe with a Soldier. Soldiers don't do too much damage, and what damage they do can be recovered with potions.
Against Brutes: Brutes do a lot of damage, but so do strikers. As long as there isn't more than one Brute in the enemy group, strikers should be able to take down Brutes quickly enough despite their high hitpoints. I would use brutes sparingly, maybe once a day.
Against Controllers: The solution for a group of Strikers is to take down that Controller fast. Otherwise, they may be in a lot of hurt. You should use Controllers less frequently, and when you do, you want to make the Strikers aware who the most dangerous enemy is. Also in most encounter templates, you'll see the Controller is a couple levels higher than the party level. I would reduce that level by one.
Against Minions: Rangers have a lot of abilities that generate multiple attacks. They may not be as good as a controller, but especially if you have two rangers in the group, they can easily cover the abscence of a controller against minions. A dragonborn in the party can also make a difference against minions. Minions don't do a lot of damage, so they are managable as long as you don't go overboard with the number of minions you use in any one encounter.
Against Large Groups: Without a leader or controller, big fights are a definite problem. Easy solution is to avoid them. Give the players a way to hit and run, weakening the enemy. Or give them environmental advantages so they can maybe drop a portcullis to separate the enemy into two groups, and deal with a smaller group at a time. Also make sure the group has some healing potions before any big fight.
So in one day's worth of encounters you can use some Artillery, one Brute, maybe one controller, some lurkers, some minions, and lots of skirmishers. Throw in the occasional Elite, Solo, and Leader, you are really not looking at that big of a difference between a normal party and an all striker party, except the controllers they face will be less potent than controllers a regular party tends to face.
For campaign considerations, like some people have suggested, try to treat them as a special Ops force. Give them more surgical strike missions, hit and run missions, rescue missions, spy missions, etc. A focused party like you have is a rare thing, so you should embrace it and expand the possibilities. Rangers, Warlocks, and Rogues can all be very stealthy, so you can easily have to whole party involved in stealth and scouting missions. They can quietly take out guards. It really gives you a chance for a unique campaign. Don't look at the all striker party as a problem, it's a blessing.
