I must have missed it when he provided me his party and class composition, could you link that information for me? make sure it contains his loadout plus available abilities.
Apparently you didn't care enough to ask the OP that yourself, Blackheifer. Niara is right - you didn't answer the OP's question: "how you used
the abilities of your party synergistically [...] simply
using the spells and weapons the game makes available to that point." It's not about whether the tactics you listed are valid or not, it's that
you didn't answer OP's question in a satisfactory manner.
I'm joining this conversation because after all these posts no one has actually addressed what the OP brought up, and I myself am curious about the solutions, if they exist, to his problems. Blackheifer, I think you've been missing Ibrahim's main point the whole time: what Ibrahim is trying to say is that
these encounters are poorly balanced. And to support his point, he listed more detailed stats such as enemy damage vs party members' health, enemy in-game stats vs rule book, etc. Not a post in here has actually addressed these details. All I've read is that "oh but this encounter can be avoided" and "you can utilize the environment and/or external circumstances" - none of these have anything to do with party composition, individual party member abilities, and synergy between party members.
I'm getting the main point here is that these encounters are poorly balanced, a point which some folks here seem to be missing, it seems. Whether the OP is accurate or not about whether a fight is avoidable or not, that's not his point at all. I don't know why you keep clinging to that as an argument against the OP.
Imagine voicing your opinion saying that an encounter is poorly balanced, and someone answers you with "but you can avoid it". By saying that you automatically acknowledge that it is poorly balanced. If you don't think it's poorly balanced, you should explain why you think it's not.
1) They automatically start with surprise ... AND they routinely violate the 5E rules in ways that make them WAY more powerful (Multi-shot with a crossbow? Exploding arrows at level 4? Infinite uses of Mirror Image?).
2) A Bulette's mean damage on a bite is 30 hp, which is more health than anyone other than Lae'zel has. It bites AFTER using Deadly Leap, so whomever it targets is instantly downed. The Minotaurs somehow get to use Gore and then Multi-attack afterwards (when they don't even have multi-attack as an option).
3) The Spectator should be a CR3 monster (and therefore fairly easy for a L4 party to deal with.) Instead, it has twice as many HP as it should, extra eye rays, more actions than it should (4 eye rays a turn? Really?).
I haven't seen a single post addressing these specific points the OP brought up. Are these duergars indeed violating 5E rules? If they are, is there a good reason for this violation? If the bulette is so deadly, how do you soak up its damage/tank it? What about the spectator, is it violating 5E rules too? And no, I don't think answering these points with "this fight is avoidable" or "but you can exploit this and that in the environment" is satisfactory.