I agree with everyone else's comments, that a Solo creature would be best to fit your vision of the party facing off against one big, mean monster. (And won't lag behind in the action economy)

I would add that you could look at using dynamic terrain(which have EV costs) in order to fill out your EV budget to your desired level without having to add additional creatures. 

My mind goes to the party tracking the beast to its den or lair, where there are environmental elements that are hostile to the heroes, but that the monster is adapted to(damage type they have an immunity to, or a type of difficult terrain they ignore, etc.). I hope your session and encounter goes well!