Hello thur Genga, I'm going to try my best to answer this. It being your first sun team and all, it's alright. But there are some things that need to change immediately.
Ninetales
Okay here's the deal Ninetales is your sun setter, you're going to need it to stay alive. Especially in the OU meta game where Rain and Sand dominate. Having an offensive Sun setter isn't very appealing to me. He needs to be as healthy as possible so you can win the weather war. Sad thing is, Ninetales is at a disadvantage with sand and rain, since Politoed can hit it with super effective damage, and so can Tyranitar / Hippowdon. I'd suggest a sdef ninetales base 100 sdef isn't bad at all. You'll be sticking around longer in the hopes of your sun staying, keep in mind the last slot is totally preference, you could even put nasty plot if you want it to be semi offensive.
- Ninetales (F) @ Leftovers
Trait: Drought
EVs: 252 HP / 124 SDef / 132 Spd
Calm Nature (+SDef, -Atk)
• Flamethrower
• Energy Ball
• Substitute
• Hypnosis / Toxic / Will-o-wisp / Protect
Venusaur
A huge threat in all sun teams, I'm going to make him one of your main offensive sweepers. I've customized his EVS to allow you to out speed common threats such as Scarfed Terrakion, And scarfed Genesect. (In the sun that is)
- Venusaur (F) @ Life Orb
Trait: Chlorophyll
EVs: 252 SAtk / 224 Spd / 32 HP
Timid Nature (+Spd, -Atk)
• Growth / Sludge Bomb
• Giga Drain
• Hidden Power [Fire]
• Sleep Powder
Arcanine
Umm, there are way better physical attackers out there than him. And he doesn't really benefit your team too much, so I'll suggest one of the most threatening Pokemon in the game. Terrakion, he'll help get Pokemon out of the way who threaten Venusaur, or he can even do well on his own. He also has great synergy with Latias, who I will be suggest later on.
- Terrakion @ Choice Scarf
Trait: Justified
EVs: 252 Atk / 252 Spd / 4 SDef
Jolly Nature (+Spd, -SAtk)
• Close Combat
• Stone Edge
• Earthquake
• X-Scissor
Forretress
Looks good, but I'd run Spikes over Stealth Rocks, you'll see why down below. I'd also suggest Running 252 HP / 4 Def / 252 SDef
Cresselia
Well, I don't know why you have dual screen support when you don't really have any set up sweepers, aside from ninetales? So I'm going to suggest you use Latias, who will add great synergy to your team. A def calm mind set should work well on this team. It has some nice solid support moves, you can get creative with the sets, maybe even utilize it as a wish passer, or Healing Wish support!
- Latias @ Leftovers
Trait: Levitate
EVs: 252 HP / 252 Def / 4 Sdef
Bold Nature (+Def, -Atk)
• Calm Mind
• Hidden Power [Fire]
• Dragon Pulse
• Recover / Wish / Healing Wish / Roost
Heatran
Heatran is really good, but this last Pokemon I'm about to suggest is going to fill the niche that this team will need to successfully sweep. The answer is Dugtrio!
- Dugtrio (M) @ Focus Sash
Trait: ArenaTrap
EVs: 252 Atk / 4 Def / 252 Spd
Jolly Nature (+Spd, -SAtk)
• Reversal
• Earthquake
• Stealth Rock
• Sucker Punch