Ok. Bowser's (pretty much) done.

Spacefaring Giant Firebreathing Turtle
Mineral Warrior Dragonborn Anthropomorphic Snapping Turtle
RHD 2 (Anthropomorphic Snapping Turtle)
LA 1 (Mineral Warrior)

Monstrous Humanoid 2/Psychic Warrior 5/Pyrokineticist 3/Thrallherd 10

ECL 20 Stats

STR 20
DEX 10
CON 22
INT 10
WIS 16
CHA 20

HD: 10d8+10d4+120
BAB +12/+7/+2
Skill Points (really not a concern. Max out Concentration and psycraft at all times. 4 ranks in Diplomacy and 8 ranks in Knowledge (psionics) are a must, and one rank in Craft(alchemy) even though we can't use it without being spellcasters will work.
Saves: Fort +16, Ref +10, Will +15
AC: Unarmored 18 (20 when fighting Dragons)

From Race
Swim Speed 20 ft.
Speed 30 ft.
Burrow Speed 15 ft.
Heart Aspect (scales up)
Monstrous Humanoid (Aquatic, Dragonblood, Earth)
Earth Strike Special Attack
Darkvision 60 ft.
DR 8/Adamantine

Class Abilities
Fire Lash
Fire Adaptation
Hand Afire
Bolt of Fire
Thrallherd
Psionic Charm
Psionic Dominate
Greater Dominate
Superior Dominate
Twofold Master

Feats:
1st: Inquisitor
3rd: Practiced Manifester
PW1: Combat Manifestation
PW2: Expanded Knowledge
6th: Psionic Body
PW3: Mind over Body
9th: Improved Unarmed Strike
12t: Psionic Meditation
15t: Speed of Thought
18t: Power Attack

Not gonna lie. I pretty much just filled in feats to suit, since I wasn't sure what kind of feats the character would have since he's...Well, fairly simple. Not too deep there. The important part is that at level 20, he has... For powers, make sure he's got at least mindlink (from Expanded Knowledge) and expansion. The rest is cake. I have some kind of legitimate layout level by level, but I don't really know that it's completely necessary or warranted here.

Spoiler: This is why we don't use Leadership
Show

His army is broken down as follows:
From his own power:
135 1st level believers
13 2nd level believers
7 3rd level believers
4 4th level believers
2 5th level believers
2 6th level believers

And 2 level 18 thralls.

Who are his thralls you ask? Well, Bowser Koopa Jr. and Larry Koopa.

Each of them have the same build going, except Psion instead of PsyWar and Cryomancer. (since, being the same race they're just on the smaller end of the spectrum. See: Dwarves Vs. Half-Orcs).

Which would mean that, assuming the same stats, each of them would have a 25 leadership score as well, bringing Bowser's total army to...

405 1st level believers
39 2nd level believers
21 3rd level believers
12 4th level believers
6 5th level believers
6 6th level believers

2 level 18 thralls
4 level 16 thralls.

Again, assuming the same stats, we now have four more thrallherds in the family: Wendy O. Koopa, Roy Koopa, Ludwig von Koopa, and Lemmy Koopa. These guys, fortunately, can still get to 10th level Thrallherd. They also have a leadership score of 25, leading to Bowser's total army being...

945 1st level believers
91 2nd level believers
49 3rd level believers
28 4th level believers
14 5th level believers
14 6th level believers

2 level 18 thralls
4 level 16 thralls
4 level 14 thralls

There's no way that the third generation thralls could POSSIBLY get to 20th level Thrallherd, so we'll say only two of them bothered to join in the family tradition while the other two focused on killifying things with magic. Those two would have, as thralls, Morton Koopa Jr. and Iggy Koopa. They'd have a total leadership score of 23 apiece, which would bring us to having...

1125 1st level believers
109 2nd level believers
59 3rd level believers
34 4th level believers
18 5th level believers
16 6th level believers

2 level 18 thralls
4 level 16 thralls
4 level 14 thralls
2 level 12 thralls

Now then, as a requisite feat, if Iggy and Morton Jr. took the Leadership feat, we'd be looking at them having a Leadership Score of around (12 + 5 CHA + 2 Stronghold + 2 Great Renown) = 21. Which would bring our totals up to

1245 1st level mooks
121 2nd level mooks
65 3rd level mooks
38 4th level mooks
20 5th level mooks
18 6th level mooks

2 level 18 thralls
4 level 16 thralls
4 level 14 thralls
2 level 12 thralls
2 level 10 Cohorts

Now then if each of THEIR cohorts takes Leadership but assuming only an average CHA of 14 (+2 mod), each of them would have a Leadership Score of, at most, 10+2+2+2 (16), so they'd raise the number to

1295 1st level mooks
125 2nd level mooks
67 3rd level mooks
38 4th level mooks
20 5th level mooks
18 6th level mooks

2 level 18 thralls
4 level 16 thralls
4 level 14 thralls
2 level 12 thralls
2 level 10 cohorts
2 level 8 cohorts.

Each of THEIR cohorts lakes leadership, same CHA mod, but they're at a Leadership score of 14, which would mean that they'd only contribute 15 1st level mooks and 1 2nd level mook apiece and 1 6th level cohort (a mook, essentially) total, bringing us to

1325 1st level mooks
127 2nd level mooks
67 3rd level mooks
40 4th level mooks
20 5th level mooks
20 6th level mooks

Now, if each 6th level believer took Leadership as a feat and considering an average CHA of around 14 apeice, assuming a Leadership score of 12 (6 + 2 CHA + 2 Stronghold + 2 Great Renown (before the war against the Mushroom Kingdom, essentially)) that would end up (with believers and followers lumped together under the name "Mook")

1485 1st level mooks
127 2nd level mooks
67 3rd level mooks
60 4th level mooks
20 5th level mooks
20 6th level mooks

2 level 18 thralls
4 level 16 thralls
4 level 14 thralls
2 level 12 thralls
2 level 10 cohorts
2 level 8 cohorts.

Total. I'm not writing all of that out in detail unless more detail is requested (And I don't want to derail the thread), but that's that.