In my humble opinion infernos are best if you boat them (meaning you jam as many as you can fit onto a single mech), and if your pilot has high gunnery, and the Multi Target and Sensor Lock skills. Bonus points if the rest of your lance has some big guns and called shot masters. The idea is to start the turn with a shutdown, then let the rest of the lance get the kill (possibly by headshot for max salvage). That only works if you've got all of your infernos on one or two mechs.
In terms of what mechs would work as inferno boats, the easy, hands down winner is the Kintaro. It can carry a whopping 5 inferno racks and a TAG with a ton to spare (so you can add a ton of armor or strip a ton of armor and carry 4 jump jets).
Other options (of varying quality) for heat locking and/or crowd control include:
- The Firestarter (6 flamers, a TAG, and 6 JJs with a ton to spare)
- The Javelin 10N (3 inferno racks, or 4 if you want to strip JJs and armor)
- The Assassin (3 infernos and a TAG if you strip JJs and a half ton of it's already paper thin armor)
- The Griffin 1N (3 infernos, a flamer, a TAG, and 5 JJs with 3 tons to spare)
- The Hatchetman 3X (2 infernos, 4 flamers, a TAG, 4 JJs, and full armor with 4 tons to spare)
- The Wolverine 6R (3 infernos and a flamer with 5 JJs, good armor, and an LBX-2)
- The Archer 2S (4 infernos with 16 tons for JJs, energy weapons, and heat sinks I guess)
Regarding the TAG, it gives the rest of your mechs a damage bonus that round, so it's nice to have but it's not strictly necessary. I think they're a good value on an inferno boat. That's the mech that will most likely open the round, so applying a damage bonus at that point goes a long way toward making up for the damage you lose by boating infernos rather than SRMs or something. Plus TAGs make it a bit easier to get a headshot kill. And they only weigh a ton.
Personally, I wouldn't bother with anything but the Kintaro, Griffin, or Wolverine. I like my inferno boats to work for heat locks, crowd control, spotting, and sensor locking, so I need them to be fast, mobile, and tough. I use the inferno mech to spot for the rest of the lance, and to dart in to apply heat debuffs and heatlocks once the rest of the lance is closer.
For inferno strategies, it's situational. as a rule infernos are great in hot biomes, good in heat neutral biomes, and not so special in cold biomes. For the latter if you want to guarantee a shutdown you need a lot of inferno tubes. Advanced versions (+, ++) of inferno launchers and flamers exist, and a bunch of those will work anywhere.
If you have enough launchers you can shut down anything in one volley, which makes it a good opener if you want to kill a hard target or set up a headshot for full salvage. If you're in a hot biome and/or your targets are already hot you can sometimes use Multi Target to get two shutdowns in a turn.
Another Multi Target inferno option is to heat up 2-3 targets so they can't shoot back the for next couple of rounds. A lot of the 3025 mechs aren't great with heat, so if you hit them with +30 apiece and apply the Burning Gel malus for the next turn they will often hold or limit their return fire. It's a good tactic against pretty much anything with a big alpha - if you have enough missiles and you're in the right biome.
In conclusion, infernos are great in the right situation, but only if fire enough of them.