Phase 1
What Thundurus needs more than anything is:
1.Entry hazards support
2.Rapid Spin/Defog support
3.Priority
The pokemon that can fulfill that rule are: Donphan, Gliscor(Lol quick attack), Kabutops, Excadrill, Forretress, Empoleon(If you want to go physical), Keckleon, Mew(of course), Skarmory and Scizor.
Phase 2
The pokemon should also have a resistance to Stealth rocks to minimize the damage taken by Stealth rocks when it enters to clean the field, a resistance to Ice would also be appreciated but the rock resist is more important reducing the list to: Donphan and Excadril, while the other pokemon are still viable choices, Gliscor doesn't have good priority, Kabutops is frail even if Thundurus covers its weaknesses rather well, Empoleon and Kecleon is rather easy to see why, Mew doesn't have a appreciated typing, Skarmory doesn't cover weaknesses or gets weaknesses covered, and scizor doesn't like having to waste a moveslot in sticky web and another in defog even if it is the most offensive of them all. I'll keep Forretress on the run because all entry hazards+Heal order+ rapid spin+ bullet punch can do work.
Phase 3
Between Donphan, Excadrill and Forretress we have to see what we want, Donphan and Forretress are walls while Excadrill is a attacker, priority is one of the things I wanted because of the lack of priority on Thundurus is bad making him struggle against priority users, having priority also gives them a way to finish what Thundurus didn't end, and this is where my struggle to choose begins Foretress is bulky, has all entry hazards and has recovery , Excadrill has 2 types of entry hazards, amazing attack being the best revenge killer and the best speed but doesn't have the right bulk, and donphan has the attack but unSTAB priority, 2 entry hazards and good bulk but lack of recovery, so I decided to cut Donphan because while his priority helped him against dragons it was unstab with only 40bp and while bulky he didn't have recovery to help withstand many blows.
Phase 4
Excadrill and Forretress have both many important features besides those mentioned above, Excadrill also has better speed and 4x resistance to rock and fantastic HP, Forretress has overcoat to take care of many spore spammers but unfortunately misses the cut because it is still set up fodder and can't do much to pokemon who he cant Toxic or finish of with Bullet punch like coil Scolipede.
Phase 5
The problem now is fitting all into 4 moves, Rapid spin and Bullet punch are some of the reasons why I chose him in the first place so I can't take them of the set and it would be stupid to not have Earthquake on the third slot, the problem now is choosing between Spikes or Rocks, Rocks are safer as you don't have to put 3 layers in order to do much damage, but Spikes forces more the use of Defog which can grant Thundurus a lovely +2 to his Bolt strikes and hitting spore spammers and some grounded pokemon harder, all around these might be interchangeable as the team building goes. I'm not really set on the EV spread or item so I'll accept suggestions:
Excadrill @ Leftovers
Ability: Mold Breaker
EVs: 252 Atk / 4 SDef / 252 Spd
Jolly Nature
- Rapid Spin
- Earthquake
- Bullet Punch
- Spikes/ Stealth Rock