CM Roar Latias does not beat Rankurusu, because no good player using Rankurusu would keep setting up CMs on a Latias. It would Psycho Shock for the 2HKO right off the bat.
Assuming max HP Reuniclus(They need the bulk,so no investment in Sp.Attack),here are his Psycho Shocks against a 252 HP/0 Def Latias:
+0 = 286 Atk vs 216 Def & 364 HP (80 Base Power): 57 - 67 (15.66% - 18.41%)
+1 = 429 Atk vs 216 Def & 364 HP (80 Base Power): 85 - 101 (23.35% - 27.75%)
+2 = 572 Atk vs 216 Def & 364 HP (80 Base Power): 114 - 134 (31.32% - 36.81%)
+3 = 715 Atk vs 216 Def & 364 HP (80 Base Power): 142 - 168 (39.01% - 46.15%)
+4 = 858 Atk vs 216 Def & 364 HP (80 Base Power): 170 - 201 (46.70% - 55.22%)
+5 = 1001 Atk vs 216 Def & 364 HP (80 Base Power): 199 - 234 (54.67% - 64.29%)
+6 = 1144 Atk vs 216 Def & 364 HP (80 Base Power): 227 - 267 (62.36% - 73.35%)
As you can see,Reuniclus needs to be at +4 to even be able to 2HKO with Psycho Shock.
How much does Lati do back?
With Dragon Pulse(The second number is Reuniclus,the first is Lati):
+0 and +0 = 256 Atk vs 206 Def & 424 HP (90 Base Power): 120 - 142 (28.30% - 33.49%)
+0 and +1 = 256 Atk vs 309 Def & 424 HP (90 Base Power): 81 - 96 (19.10% - 22.64%)
+1 and +2 = 384 Atk vs 412 Def & 424 HP (90 Base Power): 91 - 108 (21.46% - 25.47%)
+2 and +3 = 512 Atk vs 515 Def & 424 HP (90 Base Power): 97 - 115 (22.88% - 27.12%)
+3 and +4 = 640 Atk vs 618 Def & 424 HP (90 Base Power): 102 - 120 (24.06% - 28.30%)
+4 and +5 = 768 Atk vs 721 Def & 424 HP (90 Base Power): 103 - 123 (24.29% - 29.01%)
+5 and +6 = 896 Atk vs 824 Def & 424 HP (90 Base Power): 106 - 126 (25.00% - 29.72%)
+6 and +6 = 1024 Atk vs 824 Def & 424 HP (90 Base Power): 120 - 142 (28.30% - 33.49%)
+3 and +0(After Roar) = 640 Atk vs 206 Def & 424 HP (90 Base Power): 300 - 354 (70.75% - 83.49%)
Assuming Lati switched in,Latias can use Roar at around +3,possibly recover the damage,and keep up the CM race later on or just downright rape him.
That was a Latias with no investment in Sp.Attack.
Can people stop assuming Latias cannot stop Reuniclus?