In general, you can use monsters tuned for a more powerful party against a less powerful party by:
- Reducing the number of them you use; if you'd be using ten of them against a DF party, maybe only use five against your weaker party.
- Adjusting the type of challenge you assume they represent. A fight that is expected to weaken or delay but not seriously challenge a DF party might be a serious or deadly challenge for a weaker party, but that doesn't mean you can't use it, it just means it serves a different narrative purpose.
- Creating situational effects that help the weaker party.
- Adjusting the statistics for the monster.