As a DM, one of your responsibilities is to design exciting and challenging encounters for your players. The design of 4th edition D&D makes this responsibility easier to fulfill than previous editions due to the careful balancing of character roles, monster statistics, and experience curves. The DMG provides simple, but accurate guidelines on how to scale encounters and what combinations of monsters provide easy, moderate, or difficult challenges. The one detriment to these guidelines is that they are written with the assumption that your party contains at least one of each character role. When one of those roles is missing, encounters can prove much more difficult than anticipated.
If you find yourself designing an encounter for a party without a striker, you need to take some care in how you design the encounter. Strikers deal the most damage of any role and are most capable of targeting the biggest threats. In a party without a striker, fights are likely to take longer and dangerous enemies are likely to have more opportunities to use their strongest abilities. This means that, as a DM, you need to carefully consider what types of enemies you populate your encounters with.
Soldiers – Soldiers are the best counters to strikers. They have higher defenses, solid hit points, and can apply marks. The lack of strikers actually makes the soldier less effective at fulfilling its role. As such, soldiers should, if anything, be more common against parties without a striker.
Brutes – Brutes have the potential to be significantly more dangerous against parties without a striker. Brutes deal high damage with low accuracy. Usually, despite the high hit points of brutes, they are quickly killed before they can deal too much damage. Without strikers, they will live longer. Now, defenders are excellent against brutes, but can only protect against so many. A good rule of thumb in a party without strikers is to use no more than 1-2 brutes per defender in the party.
Controllers – Controllers become devastatingly dangerous against parties without strikers. Status effects have a cumulative effect on encounters and the longer controllers live, the more status effects hit player characters. In a party without strikers, controllers should all be a few levels lower than the party and rarely more than one. Elite or solo controllers will make incredibly difficult encounters.
Artillery – In most standard encounter, artillery are the last to die, falling only after all front line units fall, and usually very quickly once that happens. Usually strikers will do little to change this. In fact, of all roles, leaders tend to be the best counter to artillery, providing defense and healing until the artillery can be easily attacked. Use of artillery in encounters should be unaffected by the lack of a striker.
Skirmisher – Due to their design, generally skirmishers are used to attack strikers while avoiding retribution from defenders. This is because strikers tend to be a little further extended than other roles and skirmishers have versatile movement abilities. Without strikers, skirmishers have a much diminished role. In a party without strikers, skirmishers should be relatively common and can safely be higher level than the party.
Lurkers – Lurkers are similar to skirmishers except that they tend to target the back line party members. This can cause a lot of damage and strikers will tend to want to focus on them. Despite this, the most important character role for countering strikers is the controller role. It is probably best to use slightly fewer lurkers than normal or slightly reduced levels on lurkers in a party without strikers, but their presence will not significantly affect encounter balance.
Finally, there are a couple other things to keep in mind when you are designing encounters for parties without strikers. More than any other role, strikers tend to be able to have versatile movement abilities. This means that terrain will be more difficult to overcome and if you use significant terrain features, you should increase the experience rewards. On the other hand, since your player’s characters are already more restricted in movement, status effects like slow and immobilized will not be as devastating. Also, assuming the characters have the correct skills to deal with them, traps and obstacles will be less threatening because the players will be sacrificing less damage when taking mid-combat actions to deal with those threats.
If after following all of these tips you still find that your player characters are having a difficult time with encounters, you may want to consider lowering the hit points of enemies by about 10-20% across the board. This will help counter the lack of a striker and can be done subtly without your players ever noticing. At the end of the day, if your players are enjoying the encounters and finding them challenging, you are fulfilling one of your responsibilities as a DM.