Remember that towers are next to useless unless you also have some troops defending your base.
Being orcs, your units can take quite some punishing before they die. Try using the regular orcish grunts in several companies. When set up properly, you can use a pincer movement to restrict the elves' movement and slaughter them in short order. Two or three well-placed towers can really help in this too, if you do it right you can trap the elves right where they can hit them.
Also, when you've just been attacked it is quite safe to assume you're going to have some breathing space for the next few minutes. If half the force is still left (you didn't lose half, you managed to keep half
) use those to attack while you rebuild your forces.
Instead of defending all the time, you could try to go on the offensive where possible to keep the elves weak.
Also, if you want to be done sooner, definately get the shredders.
Oh, and if it doesn't work, just remember that I prefer elves over orcs any day and it may just have been bad advice to begin with