Prepare three things
Finish the currently useful protection before starting a difficult attempt. Identify a second position you can actually reach if the first approach becomes unsafe. Check that defenses intended for that approach are unobstructed and that a teammate knows when you plan to move. Do not choose a new expensive item because a promotional label says BEST.
Watch the first contact
Watch where the boss approaches, what it attacks first and whether your chosen position remains usable. If it bypasses the lane, stop assuming the lane controls it. If your own construction prevents movement, correct access after the run. If one exposed device is lost and all your useful coverage disappears, your next change is independent placement or fallback coverage. These are diagnostic questions, not a claim that every boss has phases, adds or a specific targeting rule.
A position-first response
Begin where you can see the relevant approach. Move to the fallback when that position is unsafe; do not wait until the route itself is inaccessible. In co-op, agree on a short call such as 'move inside' and who watches the approach during the move. Use the weapons/defenses available in your actual version. Active attacks cannot establish what the base would do offline.
Recover from the first failure
First re-establish a usable place to stand and a way to move. Then close the breach you observed or relocate the device that was obstructed. Re-test ordinary waves from a comparable state before changing several systems at once. If you cannot identify the first failure, reduce the footprint for a clearer test. Do not assume autosave restores destroyed structures, refunds costs or preserves a boss's health: those rules are unknown here.
What this cannot predict
No readiness percentage, guaranteed clear wave or HP/DPS threshold is supplied. Readiness means the preparation actions above are complete, not that the encounter is won. Boss attack animation, health thresholds and spawn frequency require actual current evidence before they can be published as rules.