In my session I have a Warlock with a Charisma score of 20 that's been using the Eldritch Blast spell with the Agonizing Blast eldritch invocation to rather spectacular results, decimating everything that I've thrown at the level 8 party.
At first I was pleased at the party's success in difficult encounters, however I've discovered over time that the consistent, high, and nigh-uncounterable force damage of this cantrip tends to trivialize most encounters, eliminating the need for creative thinking. Combine this with a School of Evocation Wizard and I've found that most encounters are swiftly concluded with an initial Fireball and a final barrage of Eldritch Blasts, with any significant damage being eliminated by their druid's healing.
Unfortunately I've found that my current approach to dealing with their combat effectiveness has led to increasingly volatile encounters. I've been forced to serve them encounters meant for significantly higher level parties, and I've found it difficult to find a middle ground where the party is challenged but not always at risk of being totally wiped.
What is a strategy that I, as a DM, can use to counter the effectiveness of the Agonizing Blast Warlock?
I should emphasize that I'm not interested in crushing my players. I simply am looking for a way to force the party out of their singular, overwhelmingly effective approach to encounters.
I'm not interested in making the Warlock useless either. This is a spotlight issue, as the Warlock's build often allows him to be the singular source of damage for the entire encounter. Unfortunately this tends to invalidate the contributions of the other players, so I'm mainly looking for a way to reduce his effectiveness so that everyone involved can feel valuable and engaged in a fight.
Bonus points if you know of a strategy that fits into a primarily undead/fiend-focused campaign.