115 HP / 105 Atk / 85 Def / 80 SpA / 80 SpD / 75 Spe = 540 BST
Justification of Stat selection:
115 HP - Can invest in either defensive stat to focus on special attackers such as Syclant and Stratagem or physical attackers such as Gyarados and Scizor
73 Spe - Max Speed 273 outspeeds every pokemon with Choice Scarf as well as +1 DD’d Adamant Gyarados. 144 EVs outspeed Adamant Tyranitar as well as needing few EVs to reach ~246 with Jolly to outspeed bulky Celebi, Zapdos etc.
105 Atk - Water / Fighting has good potential as an attacking combination, And helps to take down Ttar more easily by bypassing Special Defence boost, and OHKOs it with Brick Break with 0 Atk Evs since Superpower/Close Combat aren't likely to be used on a defensive set. 339 Max Atk makes CB viable.
80 SpA - means that it can use Special Moves if need be: 0 SpA Hydro Pump vs Skarmory = 41.86% - 49.71%. Its typing walls Gyarados well also; 0 SpA HP electric vs LO Gyara = 50.60% - 60.24% 100% 2HKO. Thus can be a surprise wall breaker with LO.
Typical EVs
108 Atk / 252 Def / 144 Spe with Impish gives:
371 HP / 273 Atk / 295 Def / 176 SpA / 196 SpD / 222 Spe
Damage Calculations using generous EVs
Timid Heatran’s Earth Power vs 0 HP / 0 SpDef =
32.08% - 37.74%
Timid Heatran’s Earth Power vs 0 HP / 108 SpDef =
28.03% - 33.15%
252 Atk Adamant CB Tyranitar’s Crunch vs 0 HP / 252 Def +Nature =
23.72% - 28.03% (4HKO at best and you outspeed and OHKO)
252 Atk Adamant CB Tyranitar’s Earthquake vs 0 HP / 252 Def + Nature =
39.62% - 46.63% (Never a 2HKO even with SR)
252 Atk Adamant CB Tyranitar’s Earthquake vs 252 HP / 252 Def + Nature =
33.87% - 39.86%
LO 252 Atk Adamant Gyarados Earthquake vs 0 HP / 252 Def + Nature =
48.79% - 57.68%
LO 252 Atk Adamant Gyarados Earthquake vs 252 HP / 252 Def + Nature =
41.71% - 49.31% (No Chance of 2HKO unless SR and Sandstorm)
Adamant Salamence +1 LO Outrage vs 252 HP / 252 Def + Nature =
79.49% - 93.55% (Not OHKOed even with SR)
Adamant 252 Atk Scizor +2 LO Superpower vs 0 HP / 252 Def+ Nature =
80.59% - 94.88% (Usually outspeed anyway)
Adamant 252 Atk Scizor +2 LO Superpower vs 252 HP / 252 Def+ Nature =
68.89% - 81.11%
CB Scizor Superpower vs 252 HP / 252 Def +Nature =
39.63% - 46.77% (3HKO at best)
CB Scizor Superpower vs 0 HP / 252 Def+ Nature =
46.36% - 54.72% (3HKO)
Timid Stratagem’s Thunderbolt vs 0 HP / 0 SpD =
64.15% - 75.47%
Modest Choice Specs Paleo Wave vs 0 HP / 0 SpD =
35.04% - 41.24%
LO Timid Stratagem Weather Ball in Sandstorm vs 0 HP / 0 SpD =
32.61% - 38.54%
LO Timid Stratagem Paleo Wave vs 0 HP / 0 SpD =
27.49% - 32.61%
Timid Syclant +2 Ice Beam vs 0 HP / 0 SpD =
46.09% - 54.18%
Timid Syclant +2 Ice Beam vs 252 HP / 0 SpD =
39.40% - 46.31%
You can always invest more in Special defence to help against Stratgem and Syclant.