Non-Int-based death attack (aside from marrulurk and Marchosias)

With 3rd-party sources available, one can reduce the tower shield's armor check penalty from −10 to at least −1...

• The armor and shield special ability maneuvering (Complete Book of Eldritch Might 163) (+1 bonus) and greater maneuvering (ibid.) (+2 bonus) reduce an armor or shield's armor check penalty by 2 and to 0, respectively. Yep, to zero. Straight up.
• Alternatively, for Pathfinder there's the celestial shield (Ultimate Equipment 131) (13,170; 7 lbs.), which weighs half the amount of a typical heavy steel shield but has no armor check penalty. After subtracting the cost for the masterwork heavy steel shield itself and the total +3 bonus, the celestial shield's remaining abilities ("It has no armor check penalty or arcane spell failure chance, and it allows the wielder to use feather fall on himself once per day") can be extrapolated to cost 4,000 gp. Talk to the DM.

But using exclusively Wizards of the Coast material to do this is, I think, impossible.

...But, sadly, nothing I've found permits making a shield bash with a tower shield

A tower shield (PH 123, 125) (30 gp; 45 lbs.) specifically can't be used to shield bash (PH 125). In fact, one can't even install on it shield spikes (PH 125). (True even in Third Edition, Plot and Poison being an unupdated Third Edition text.)

Green Ronin Publishing's Plot and Poison: A Guidebook to Drow, part of the publisher's Races of Renown line, lists as the entire benefit of the feat Shield Mastery as

Choose a type of shield (buckler, small, large, or tower). When you use that type of shield, its armor check penalty is reduced by 1. Armor check penalties cannot be reduced below 0. This feat may be chosen as a fighter bonus feat. (96)

Hence, one can't make a shield bash with a tower shield because of the feat Shield Mastery. Further, the benefit of the feat Shield Specialization is

When you make a shield bash attack with a type of shield selected for the Shield Mastery feat, you can regain the shield's armor bonus to your AC as a free action. If you choose the buckler for this feat, you can make shield bash attacks with it that deal 1d3 points of damage (×2 crit) and do not suffer the normal −1 penalty on off-hand attacks, whether with weapons or shield bashes. In addition, you can regain the buckler's AC bonus after making an attack with an off-hand weapon. This feat may be chosen as a fighter bonus feat. (96)

Thus, even if the tower shield is picked for use with the feat Shield Mastery, the feat Shield Specialization doesn't enable a tower shield to be used to make shield bashes. The text doesn't allow doing so in the same nonstandard way that is allowed with the buckler.

The benefit of the feat Double Shielding says in part that

You can make shield bash attacks with your primary hand, incurring no off-hand penalty, but you lose the armor bonus of that shield until your next action as normal. If you use two bucklers and employ two-handed weapons or ranged weapons that require two-hands (such as all bows and heavy crossbows) you incur a −2 penalty on all attack rolls. (91)

Most would read that benefit as Since one can't make a shield bash with a tower shield, the feat Double Shielding doesn't change that. Now, it's totally cool if the DM reads the benefit You can make a shield bash attack with your primary hand as also implying And the DM must concoct house rules for shield bashing with a tower shield, but that's not a reading I'd support. That's like taking a feat saying You can make an attack with a greatsword that does 6d10 damage or whatever and claiming that means a greatsword appears in the character's hand whenever he uses the feat and makes an attack.¹

I spent some time searching through primary, secondary, and tertiary Dungeons and Dragons, 3rd Edition and 3.5 books, and making a shield bash with a tower shield seems impossible. Undoubtedly, there's some crazy publisher during the OGL boom that published a Tower Shield Bash feat or a slamming shield special ability or whatever, but I couldn't find a way to make a shield bash with a tower shield.

¹ Okay, not really, but no analogy is perfect.

If you can meet the prerequisites of prestige classes by your previous gestalts, than it is reasonable to take Beastmaster as soon as possible. I assume you are going to finish your Convoker advancement, so the character would look like that:

Commoner 6 // Convoker 6 // Beastmaster 1

At the first level Beastmaster provides boost to your effective Druid level, so it will be a companion of 4 level Druid from the spot. I dont realy think you'll very much need any more levels in Beastmaster as it stacks with any companion advancing classes, and they have better class features.

From now on I see two options:

1. Take Druid for spellcastng, if you have room for it in your actions. Five levels will give you access to highest level spells even without that level lost due to Beastmaster. You also will have room for adding more prestige classes at those five levels later, as Druid is a regular class. Finaly, as a regular class, it has no prerequisites. It results in:

   Commoner 6 // Convoker 6 // Beastmaster 1/Druid 5

2. Stack some other prestige and maybe regular classes with nice class features wich progresses your animal companion. I'd consider Wild Plains Outrider, good class with quick advancement and ability to make a full attack after mount's single move. Two remaining levels may be of practically any class wich progresses your animal companion, probably those of a Beastmaster or even some regular class whose class features you like. It results in:

   Commoner 6 // Convoker 6 // Beastmaster 1/Wild Plains Outrider 3/xxx 2

I can't realy wrap my mind around Animal Ally feat mentioned by Chemus. I feel like, depending on your DM's judment, it may be quite usefull, but when to take it is a question I can't answer. Beastmaster and Wild Plains Outrider are both from Complete Adventurer.