Counter and mirror coat are generally unreliable ways to deal damage because they take up 2 move slots, and Deoxys has very low HP. After opponents have seen you use those moves once, they can easily avoid damage by switching out or using status moves. They're even worse in doubles because the opponents can just attack your other Pokemon. Spikes is also pretty useless in 3v3 singles and 4v4 doubles because the opponent has no more than 2 options to switch to, so they're usually going to use a move regardless of spikes.