Big D only spawns once you take a couple steps towards any “pillar” in the corner.
Scout the level and note wherever has lots of water (which will soon be electrified).
As you should know by now, it becomes invulnerable during the fight.
The pillar making it invulnerable is telegraphed by the direction towards which the sparks will fly.

If you have one or more honeypots:
Starting with the most annoying corner to get to, step in towards the corner as little as you can and throw a honeypot at the pillar. If you don’t go too far in, DM won’t spawn yet and you can repeat for other pillars if you have the honeypots.
This won’t do anything for now, but once the bot is invulnerable, the bee will kill the pillar, canceling the invulnerability without you having to walk there.
Now, you likely won’t have 3-4 honeypots on most runs, but even if you only have a 1-2, that’s still less running around.
If you’re lucky or have enough honeypots, you don’t even need to chase the pillar at all.

If you do need to destroy a pillar manually, cutting through the middle area (DM will have more trouble following you there) could buy you some time.
Other useful things to get to a pillar faster:

• yellow seed
• haste potion
• teleport rune stone

The lightning bolts revolve around the pillar, try and time it so you don’t get zapped too much.

When done, I usually go out the other side because the way I came in is likely full of poison gas by now.

Mitigating damage:

• avoid electrified areas
  • you take damage from it (bad)
  • it gains shield from it (bad)
• when it tries to drop the ceiling on you, move to a non red square to avoid being pummeled while paralysed
• in melee, an earth root (brown seed) can help you tank some (Beware that it’s canceled by moving)
• if you’re taking too much poison damage, maybe use a purity potion, if you’re still in earth root, you’re probably better off trading hits than moving
• kiting it can avoid a little poison damage too, that is, if you’re not currently using the earth root to mitigate the physical damage (move away, attack, move away, attack, etc).
• if the only damage you’re taking is the poison, you’re fine

Other random things of various use that you may or may not have…

• Liquid flame can provide you with a bit of damage over time (it’s immune to poison gas, but not to fire)
• Wand of Regrowth can root it in place (or give you more turns of fire damage)
• Similarly, a potion or a wand of corrosion could provide damage, but it usually moves too much to be effective unless you can also root it, or vertigo it (by throwing a levitation potion at it or throwing a stormvinein its path) Do NOT get caught in your vertigo or corrosion.

PS: lower tier weapons and armor don’t really keep up at higher levels. Consider holding off your first few upgrades (making the first levels harder) to upgrade better tier stuff (making higher levels easier, somewhat)
It’s… a balance. Holding off too much isn’t always better either as you then have to burn through your other resources to survive.

Good luck

Definitely my least favourite boss. Can’t give any advice as it is usually a toss whether my fights with it go well or not.

DM300 is easy next to black goo even with badder boss. For me tengu is harder and annoying since he gives damage every turn.