Stumbled upon an odd strategy that I like to call the 'dragon cannon'. It's based off the idea that the dragon's natural magical attack and knockback allows it to push around the magic immune plants. In other words, these plants actually become a source of damage for a dragon.
Since no strategy is worth its salt unless it can take a vicious dungeon, I attempted this in the only one that allows you to spawn plants around the bosses: vicious halls of steel. I prepped the earthmother (obviously), a bear mace for extra knockback, the transmutation seal/sensation stone massive cp combo, and a smugglers dungeon just because (no extra potions).
To my surprise, it worked like a charm! Admittedly, I got pretty lucky: I found weytwut pretty early so I could avoid getting trapped by the earthmother's plants. I bought entanglement out of the gate and the alternated between shots of vine form and greenblood to fill up the map with plants as well as alternating between stoning slowed popcorn (or plants) and killing the highest level monsters I could to maximize experience.
Then, I got extra luck and found Binlor! I blew through the rest of my piety, took plantation, and converted in. From there I took stone fist and then a couple of shots of stone heart, not because I was worried about resistances but because I wanted to clear out the walls to make paths I could push plants along. As icing on the cake, the secret subdungeon had a second bear mace! When all was said and done, my knockback was at 220%.
At level six I began push plants into count blahblah and got him down to < 50 hp, at which point I finished him off with fireball. I was about out of plants, so I took on the indomitable the old fashioned way, knocking him into a long line of walls and fireballing/discharging burning stacks when he was down to 1 hp to peel off the death protections. I figured with vine form I would be able to plow through him no problem, but I forgot that I had accumulated something like 15 curse stacks while clearing out non-magic immune paths to send my 'dragon cannonballs' down. I didn't have nearly enough popcorn to clear that, and I thought that might be the end, but fortunately the regular subdungeon I spawned was the Pa'an labyrinth, which was pure black space for a dragon that can see the hellbois before revealing them. In the end, I pushed the Indomitable up against a warlock and just kept smacking until he went down. Success!
As I mentioned earlier, most of the vicious dungeons are set up so you can't spawn plants around the bosses, making this a useless strategy for anything but the vicious halls of steel. You might be able to pull it off in Naga City, but I imagine that would be much harder since you have to start the plant spawning from scratch when you descend to the boss level. If it's possible, it'll probably take a better player than me to pull it off.
Still, this proved a fun and novel way to clear the VHOS with a half-dragon (and it makes the southern swampy levels a piece of cake).