Well actually... Omastar blows 80% into Ferrothorn that try walling it, can revenge Exkiller at +2 since it OHKOes it with HPUMP after rocks to those that invest 132 evs in HP while ARceus does a trollish 40% percent back. In fact all the Pokes you listed don't even want to switch into Omastar after (not while or before) it smashes lol. Omastar gets Shell Smash which enables it to sweep really nicely. If it dies in the process Deoxys-a can finish the job up. I think thats what qualifies it a spot over anything else. Mixdra does the same but we can replace Oma over terra while still not fearing Arceus sweeps :P.