okay. just watched a few lp on the game found out my game glitched for the 2 boss. there meant to be a chain with "target" on it but it nevers pop in my run. so redownloading the game -_-

mission 0 has the 2 arms and head.
mission one has the 4 cannon pods, then 8 parts that circle around it, then the 2 guardshells in front, then 8 spikes that stick out of it, then the main body.
mission 2 has the 2 shells, then the 3 cannons, then the weapons bay (destroy the bay FAST after destroying the three cannon) then the the shells and horns, then 3 more cannons on another bay (these cannons respawn immediatly so keep track of which ones you destroy) then the 2 side engines, rear engine with 2 cannons (1 on each side of it that also respawn immediatly) and the top part. finally the main body after
mission 3 has the magnet, the 2 tessa chains on each side of it, then the 2 dump cores, then the main body. the whole time throwing doen the sunny circles.
Mission 4 is a boss rush. first boss is one part, just kill it. second, all the spinning things around it again, 3rd, the cubes, 4th has 4 small planes and a big one. the big plane has 2 big wings, 1 small square type part on both sides of the main body, and the main body. last boss has 6 searchlights, 2 horns, 4 engines, then the main body.
Final mission only really has one part that just changes forms.
so all together now we have
0-3
1-23
2-20
3-7
4-28
5-1
total-54
the achievement progress only goes up to 43 however and for me it says i have 42 out of 43. the only thing missing is that tessa chain on the second boss when your destroying the 2 guardshells before the 8 spikes come out of it. on the ps3 the tessa chain is there and i have the trophy. but on steam, after reinstalling it as well, theres no tessa. so im not sure if there is suppose to be one on the steam version, and im not sure what parts actually count and what dont, seeing as how i counted an extra 11 parts.

I don't know what's with the chain that's on that boss in some videos (which are probably of an older version of the game), but it's not in the current version of the game as far as I'm aware, and it's not necessary to get the "Destructive" achievement either, as I got that achievement within a few plays of the game when I was first starting out, despite there being no chain on that boss. There's probably a different boss part that you're missing somewhere. It would be nice if it was easier to figure out exactly what boss parts count and what doesn't...

Without me, folks. *huge grin*
I have been trying for way too long to figure it out to no avail.

I'm not gonna mention every one of your small mistakes in your analysis since I did so in another topic, but it seems you don't need to destroy the 3rd mission's magnet, or a specific number of cubes in the 4th mission. Damaging them without destroying them works.

The 4th mission's second "boss" also seems to be a single part, since destroying these rotating modules is the only way to damage the boss.

And I don't have any chain either during mission 1's boss, yet got the achievement one random day.
So... I can't be much of help either. And I wish I could share the replay where I got the achievement, but erased it by mistake when I sorted my replays out not so long ago. If only we could rename our replays...

Ancient bump but it's not like this forum has many topics anyway so no need to create a new one.

Add me to the 42/43 club though. I have a replay where I destroyed all of the below:

0:
1 Left Arm (by bombing Right Arm)
1 Right Arm

1:
4 (all) Gun Pod (turrets) in the first form
3/4 (hard to tell) of the 8? Guardshell during the short spinning form
1 of the Guardshell during the form with two (destroys the other one and proceeds)
8 (all) of the Mishwing (blades) in the last form

2:
2 (all) R'lshell
3 (all) Cannon Pod
1 Weapon Bay (to proceed)
2 (all) R'lshell
1 R'lhorn (destroys the other one and proceeds)
3 (all) R'lhead (to proceed)
3 (all) R'lshell (to proceed)
2 (all) Cannon Pod
1 R'lengine (to proceed)

3:
1 Super Catch Beam
2 (all) Dump Core
4 (all) Tessa (one after it had started transforming into the last form, but was still destroyable)
2 (all) Dump Core in last form too (watching videos, these are likely the same dump core as the first form)

4:
6 (all) Lhasa (the search lights)
2 (all) Himalaya (the horns)
4 (all) Himalaya (the thrusters)

In fact I destroyed everything that could be! So I'm thinking it has to be one of a few things:

Possibility 1: Right Arm needs to be bombed directly. I did damage it with a few Leaf shots, but it was destroyed by bombing Left Arm instead.

Possibility 2: You need to damage both Guardshell in the third phase of the stage 1; watching the replay, it's possible I didn't hit it once, so even though it's always destroyed automatically, maybe it needs to be damaged to give credit.

Possibility 3: All the Tessa in stage 3 need to be destroyed before it starts transitioning to the next form, and even though they can be after that, they might not count.

Possibility 4: The achievement disables pausing (I always need to check which of the R'lyeh parts to destroy first).

Possibility 5: The achievement is semi-broken and doesn't always unlock even when you do it right.

Edit: Got it!! Sadly I can't rule out any of the above possibilities since the next run tried all five of them (bombing Right Arm, hitting both Guardshell in stage 1, destroying all the Tessa before any Dump Core in stage 3 and without pausing the whole run).