I don't necessarily use it as my dragon counter (though it walls Garchomp completely) as more of my physical wall. HP Ice is used due to the amount of Latios/other Dragons that switch in to try and wall me. And since I only run one Chlorophyll pokemon (Venusaur) it doesn't add any extra weaknesses to my team. Since it is a sun team the Regen/Wall part surprises a lot of people, as they think to switch in and eat a Sleep Powder to the face.
Tangrowth over Cresselia for me, as with no investment it hits harder, and the typing is better for physical walling then Psychic. Blissey is used as my Special Wall, and together Tangrowth and Blissey form an impressive defensive core that is hard to break. Tangrowth also boasts being able to go toe to toe with any sand sweeper and winning so....
Also I have recently tried Eruption Flash Fire Typhlosion and I am extremely impressed with it. Choice Specs Eruption has allowed me to take out half of the enemies team without a flash fire boost, and when it does get it it becomes a monster. Solar Beam/Focus Blast/Overheat provide coverage moves. Overheat is for when I start to run low on health.
That makes more sense lol, I guess I was seeing your choice out of context. I'm still not sure I'd use a Regeneration Tangrowth on Sun though - I'd probably do a fast SubSeeding or Synthesis variant with Chlorophyll if I did use it. It does exacerbate your weaknesses if you have Venusaur, however, but may not do so to a awful extent, especially with Blissey.
Since Cress has Reflect and greater physical defence and HP to start with, it can invest less in them and more in SpAtk to bring it's offences up to par (especially as Ice Beam has better BP than HP Ice). Reflect to some extent offsets its poor typing, which is nice. Basically Im just saying this as I just don't want others to dismiss Cress out of hand as a physical wall, though it may not suit your team well.
Tyhplosion is something I've tried quite a lot, but I could never really make it work. Coming in without being statused or taking damage is pretty damn difficult, despite being able to force most steels out, and this makes Eruption drop in power to a point where other Fire type's Fire Blast would hit similarly hard. Do you utilise anything in particular to help him get in, and if not what are his best free switches he can fire off his Nuke-esque Eruption on?