May I add -
Never Give Up, Never Surrender! (Yes a movie quote but it's a good principle!) - I've lost my mage in the first round and gone on to win. This has happened not only in group fights but also one on one sessions against another person. With the element of chance this game is very fluid so fight on until your last life point ebbs away!
Move, FIRE, run - move your mage out of cover, fire off a spell or two, run them back behind cover! You have to be careful as your sprint distance is variable each time but it works a treat if you only have to run a square or two
Eye Spy - the fastest way to check what square you need to be on to target an enemy unit is to click on the Line of Sight icon (looks like an eye) and then click on the ENEMY UNIT in question. This immediately shows you which squares it can see - AND be seen from. Much faster than clicking around at random.
Run and launch - Move your mage (even sprint as well if you want) then summon a creature - which can itself move before it attacks! This means you can hit enemy creatures a really long way away...I might get the exact number wrong here but let me see...move 5 squares, sprint up to 4 is it? Then cast in the square adjacent and then that creature runs 5 and can now attack...so a hitting range of around 14 or so!
Try it out - it's nasty!
In Free for all, definately never give up, the other teams will take each other down to just guardians eventually too. To play the best with only guardians in a FFA, team up with another person whos weak and team up on the big guy. Sure, If I have my hero alive and you don't 99/100 I never lose, its too easy to stay outta reach. But with 4+ units coming at me you just never know.
And actually the summoned units major strength is, they can hit a enemy unit that is not in Line of sight from any other attack.
Anyone who plays me knows my first basic strategy mixes "run fire and cover" with Run and launch. Basically causing chaos in another players backfield with a summon (forcing him to spend time with that and not moving forward in the game) and returning my mage behind a wall with a sprint so he isn't in LoS.