There are two general 'strategies' for this that I believe are the easiest.
The first is using traps, which can be very easy to do if you've spent points in them. You can lay simple proximity mines next to the spawn points, and that can, if upgraded, mean that you'll barely ever see leak, if ever. Spike traps can kill any of the leaks that come through.
For the 1st wave I would recommend putting the mines on the stairways leading up, as you can get more use out of them by covering choke points.
By the 2nd wave you should make sure you have the spawns on the top left and bottom right covered with mines. Repair traps with any leftover mana.
On the 3rd I would suggest spike traps leading from the top left and bottom right spawns back to your crystal. Make sure they don't set each other off. Repair traps with any leftover mana.
4th wave should just be laying down some spike/mine traps by your crystal, make sure that if anything leaks through your frontal defences it won't deal much/any damage to your crystal.
5th wave, make a few more traps, gas traps can help because they slow the advancement of enemies so as to let your traps reset, so I would suggest a few of those wherever you can place them.
The second would be using traps as a support rather than as your primary damage dealer. You can use proximity mines, because while weak they still deal damage to large areas, gas traps will cause stunning that you can use, to great effect, against just about everything. You can easily kill/weaken anything of note while they are stunned with gas traps, and then let the weaker/weakened remaining enemies walk into proximity mines. Again, Spike traps can greatly help if you have leaks with large amounts of health, as while they only hit one they are a very damaging trap.
For the 1st wave I would suggest gas traps on the stairways leading up, and I would suggest laying multiples, just in case.
For the 2nd you should expand gas traps further down, to near the spawns, and cover the top left and bottom right spawns. Intersperse a few proximity mines as well. Repair traps with any leftover mana
By the 3rd you should probably make a few spikes by your crystal, and upgrade gas traps around your crystal as well. Repair traps with any leftover mana.
4th wave you should upgrade traps or make more, depending on whichever you prefer, although I suggest upgrading.
5th wave, stay alive, invest all mana into damage dealing traps.