Assuming this is Black/White, regardless of your tag being Pokemon Online which is a battle Simulator online, about Competitive Play and not in game. Look at my link to see what it is if you don't know. Just adding.
Ok, so you get Deino at Level 38-39. It can already learn some pretty solid STAB's. Dragon Pulse and Crunch. Now since you're training for the Elite Four, you should get your Pokemon to around 45-55 to go through without much trouble. There are some Woobat's around the place, giving some great starting EXP to get around level 42 or so. Once you have reached these levels, go to the Elite Four and give Deino the EXP Share. This is the best and most recommended way of training any Under-Levelled Pokemon.
Well, this is all I can recommend. Hope I helped you.