Flameblast
Now this is what I call the best spell in this game. Here is the tooltip:

And here is the result with proper scaling:

At first, I thought this spell was far too cheesy, however, as it turned out later, that is exactly how it should be for a fire mage at high levels — either you blow them up, or they blow you up.
I also scaled up my fire shotgun — decreased the area of effect in exchange for increased damage, added duplication upon casting, and maxed out the speed — the result was a sort of flamethrower for close combat. I filled the remaining slots with buffing auras and set off to storm the fourth act.
Inferno
I finally solved all the inventory problems and could afford to collect only yellow gear and above. There were more than enough various scrolls and other junk, and I also obtained several legendaries, yahoo!

An ideal amulet for a mage, which also reduces the cost of gems requiring other attributes, allowing me to use an additional stone to blind monsters upon attack.

A funny ring that significantly strengthens the mage if they are on fire. Overall, a very useful item in the fourth act, and generally against many groups of enemies; considering my fire resistance was already around 80%, there were no problems with the damage taken, but the advantages are quite noticeable. And in general, a great addition to my explosive build.
That's how I set off to conquer the gloomy dungeons of the final act, and I was somewhat disappointed. Added in the penultimate update, the act was incredibly boring and short, and it didn't take much time to complete. Among the interesting enemies, I'll only mention two — the final boss and, of course, the general of the lava worms.
Malachai

An excellent opponent for a final boss — moderately difficult, but quite manageable. Among the dangerous abilities are randomly spawning blood spheres and tentacles that prevent entering the melee zone. I died to him once (can't go without that, well), and out of absolute stupidity — I got carried away breaking his protective barriers and didn't notice I was stuck in a poisonous swamp — as usual, well.
light version of the fight; I won't be uploading the hour-long walkthrough video on merciless.
The nastiest creature in the world™
Nothing foreshadowed trouble. Another platform towering over a lake of glowing boiling magma, the quiet whistling of hissing elementals around — and then IT appears. And although the pic is from D3, overall there weren't many specific differences between them.
The trick was that this worm appeared in a glade, the path to which was only possible via a narrow pass between charred mountains. When this creature noticed a bravely running mage ahead, it immediately summoned four immortal copies that blanketed the passage with needle-like spikes at absolute machine-gun speed. The first time I kicked the bucket without understanding anything; the second time I died already realizing what was happening; the third time I tried to jump to the cursed worm using a portal, managed to blast a third of its HP with a shotgun shot, and headed back to the afterlife again.
The fourth and fifth times I tried to pelt it with meteor rain from beyond visibility, but the pest burrowed underground and healed. From the sixth attempt, I finally managed to hit all possible crits and procs and destroy the abomination in one shot, and the summoned needle copies dissolved right before my eyes, though the feeling remained unpleasant. To encounter such an ambush on hardcore — a guaranteed character rip is ensured.
Final
One way or another, I finished the game and successfully unlocked access to the second of the three difficulty levels, where I'll be heading soon.
I forgot to mention — the game has occasional server lag problems (which are definitely not local glitches), because of which today, for example, there was almost no possibility to play. However, as reliable sources say, such cases are quite rare.
Either way, the easy half of the journey is overcome, and stopping now would be like turning back!
Results: day 3-4
Playtime: ~5 hours (29h total)
Character Level: 51
Experience Gained: ~66.000.000
Death Count: 7
Game Modes: Normal ✓