If a user predicts a solar beam, they are absolutely fine switching in a hippowdon to change the weather to sandstorm, and sit there and watch as solar beam just sits there and charges for a turn.
Of course they have to switch out again to dodge the solar beam the next turn, unless they run protect (like me).
Actually, after running calcs on Solarbeam's power out of Sun, you really don't need to switch out if you don't mind taking a hit too much and really want to KO. All weather pokes easily survive the halved Solarbeam and can KO back. Posted the calcs in the Solarbeam thread if anyone wants to see.
I'd suggest Earthquake over Leech Seed on that suggested moveset just for Heatran. Tangrowth's one of the few things that can beat it in Sun. If you Amnesia up as it switches in, you can easily survive one move barring a crit to smack it's balloon and deliver a quick EQ to finish it.
Personally I'd run HP Fire/Leech Seed/Amnesia, with either Synthesis or Giga Drain in the last slot. Heatran is of course an issue, but generally Sun teams have at least one effective way to deal with him given how badly he walls Fire, Grass and Poison moves, so I let teammates deal with him and cover everything else.
I have tried this set, but somehow I feel it isn't as effective as Regeneration Tangrowth (the Subs really help as a lot can do over 25% to it, as does Regen) even in the Sun. Chlorophyll Subseed sets have been doing okay for me as a bulky alternative to Jumpluff, however (again though lack of Regen hurts).