You need speed investment to outspeed Celesteela since it's a base 61 and Incineroar is only base 60. If you want to outspeed 0 speed neutral nature Celesteela you need at least 12 speed (take it out of Attack, it doesn't really change your KOs too much). If you want to outspeed 252 speed Jolly Marowak-A (which some people do run, hilariously enough!) you need 212, that's the range of options you want to consider though I wouldn't recommend investing that heavily. 28 speed (which gives you a stat of 84 at lv50) will cover you against the 132 EV Marowaks that are built to outspeed neutral/0 investment Celesteela by 1 point as well as the Marowaks/Celesteelas that run an extra point of speed to get a leg up in that optimization war.
I've been using this spread on Incineroar to some decent success
Incineroar @ Incinium Z
Ability: Blaze
Level: 50
EVs: 176 HP / 252 Atk / 80 Spe
Adamant Nature
- Flare Blitz
- Darkest Lariat
- Taunt
- U-turn
With this spread, Incineroar ends up with 90 speed, which is more than enough to outspeed Celesteela and in the very rare instance, a +speed nature Marowak with 132 Speed Ev's. The standard Celesteela Sets are easily dealt with:
252+ Atk Incineroar Flare Blitz vs. 252 HP / 0 Def Celesteela: 204-240 (100 - 117.6%) -- guaranteed OHKO
(204, 204, 206, 210, 212, 216, 216, 218, 222, 224, 228, 228, 230, 234, 236, 240)
Whereas Heavy Slam:
252+ Atk Celesteela Heavy Slam (120 BP) vs. 172 HP / 0 Def Incineroar: 51-61 (26.5 - 31.7%) -- guaranteed 4HKO
(51, 52, 53, 54, 54, 54, 55, 56, 57, 57, 57, 58, 59, 60, 60, 61)
As far as still dealing with Alolan Maowak:
252+ Atk Incineroar Darkest Lariat vs. 124 HP / 0 Def Marowak-Alola: 134-162 (88.7 - 107.2%) -- 37.5% chance to OHKO
(134, 138, 138, 140, 144, 144, 146, 146, 150, 150, 152, 152, 156, 156, 158, 162)
252+ Atk Incineroar Malicious Moonsault vs. 124 HP / 0 Def Marowak-Alola: 288-338 (190.7 - 223.8%) -- guaranteed OHKO
(288, 290, 294, 296, 300, 302, 306, 308, 314, 318, 320, 324, 326, 330, 332, 338)
while Earthquake is still a 2HKO
252+ Atk Thick Club Marowak-Alola Earthquake vs. 172 HP / 0 Def Incineroar: 148-176 (77 - 91.6%) -- guaranteed 2HKO
(148, 150, 152, 154, 156, 158, 160, 160, 162, 164, 166, 168, 170, 172, 174, 176)
He's also really great at dealing with Aegislash in all forms since he outspeeds it handily:
252+ Atk Incineroar Flare Blitz vs. 252 HP / 0 Def Aegislash-Shield: 146-174 (87.4 - 104.1%) -- 25% chance to OHKO
(146, 146, 150, 152, 152, 156, 156, 158, 158, 162, 164, 164, 168, 168, 170, 174)
252+ Atk Incineroar Malicious Moonsault vs. 252 HP / 0 Def Aegislash-Shield: 218-260 (130.5 - 155.6%) -- guaranteed OHKO
(218, 222, 224, 228, 230, 234, 236, 240, 240, 242, 246, 248, 252, 254, 258, 260)
My biggest issue so far with using him is providing enough speed control and support so that Incineroar and Buzzwole can succeed. I'm thinking maybe twave porygon 2 but I'm all for hearing any ideas on how to make this better.