Mobility is the key. Hit and run, use your abiities smart and whatever you do, don't waste energy. That said, don't be so overly cautious with energy that you never use it, but don't spam rok strikes on single guys or summon dreadnoughts when you already have one. You should already know the effectiveness of blitzing one entrance at the start before everything merges into one massive crapstorm in the middle. A great stratergy is to kite your enemies around the outsides of the board, turning every once in a while to fire of shots etc.
Wave 20 is terribly difficult and it's annoying to fail after you've gotton so far. Once it starts, try to link up with your teammates as soon as possible, negotiate an entrance beforehand and try move to it around the outside of the board, as fast as you can. A farseer's group teleport can certainly help to move teamates rapidly around the board.
I do recommend a suppression weapon in wave 20. And all the minions you can muster. Forms of confuse can make a great impact as well. Finally, superabilities; Orbital Strikes, Roks, and Eldritch Storms can truly turn the tide of all the battles from wave 17 onwards, they are espacially deadly when coupled with suppression or knockdown weapons. Remember however, whatever you put on your hero will be thrown back in your face at wave 16.