I would say, first level go AOE, just get your AOE spell and farm creeps.

If you are rook, go hamer slam, kill creeps change lanes always go against the creeps and forget about the enemy DG, at level 3 with hamer slanm and archers theres no other dg that can actually go against you without risking dying.

Since the above part is a little to simplistic and general let me try to re-frase. I belive what you are doing wrong is let others harras you, if someone atacks you fall back to towers and atack them, of course that depends on the map and DG, lets take for example rook since you said you like him and lets take the map prision, now in that map you can really fall back without losing to much ground so what you need to do is just put 2 towers on the 2 lanes and go around hammer slaming, this aplies to any map, yu get exp from the kills of your towers so use that to gain exp by changing lanes and setting towers in any place where creeps should be.

look me up in the chat and we might play a game, im not an expert but i can show you a few things with Rook, TB and Regulus.

If you're playing as the rook, get the towers, and start raising towers near where the creeps do battle with each other. Then go start capping flags - this gives heaps of xp early game. Harass enemy dg's around the flags and when they farm creeps, but don't die - don't stray too far from your own towers. If an enemy dg comes to attack you, raise a tower then hit him a few times - at level two grab the hammer slam, use that on dg's, don't waste mana casting it at creeps.