WELL.
Tunga spawns in one of two modes.
In mode 1, he spams spells constantly, rarely melees (if at all?) and is nearly impossible to trigger.
In mode 2, he mostly melees and casts spells occasionally.
Chainspell causes the mode switch.
If no one has fought him and wiped to him after triggering a chainspell, hell chainspell 3 times: at 75%, 50% and 25%.
If you stun him just above a trigger HP%, then quickly drop his health below that amount (usually via a WS), he will
not chainspell for that %. He must be stunned when his HP drops below that % or he will use it.
So basically, if he spawns in melee mode, just keep him stunned at the proper times and he'll never chainspell, never switch to OMGWTFCASTING mode, and he's easy as ... me after four or five drinks.
If he spawns in magic mode, zerg him down to the first chainspell %, hope you survive chainspell (buy a fool's drink with dominion credits maybe), and then keep him from chainspelling again!
Hope this helps!