TANKY
my build gives something everything defensive and a little portion of damage. it makes you tank in late game.
Why spirit visage? your passive.
Why ruined king? survivability, and slow.
Frozen Mallet for health and slow.
Randuin for health, armor and slow.
and the Atma closes the build with armor and some ad, its a satisfaction everytime you end with it.

DAMAGE
it gives some health, survivability with life steal, and some critical strike chance, dont forget that Rabid Bite can critical strike, and the necessary armor penetration.

Rabid Bite is the first spell to power up, do i really have to explain you why? it steals AD, it makes you do dmg and makes the enemy loose it, come on man... and for second the W it makes you run faster and attack faster, whats better to engage a teamfight or running toward escaping enemies? then the slow, very useful but to power up last.

Just spam all the spells on the nemy when 1v1! but, for the ulti, wait for him to rip away some of your health, so u can steal, and do it in combo with the ulti. SPAM THE BITE AND DONT FORGET CONTAMINATE, a lot of people does.

IN TEAMFIGHT
Easy. use the slow on the enemy first source of damage, usually the adc or the apc, depending on who's feeded.. but try also to make it plug the escapes/slow all the enemy team. depends on the situation!
the rabid bite on the adc/enemy bruiser, who does phisical dmg and are more dangerous, obviously.
USE THE ULTI ON THE ENEMY TANK, dont forget that IT STEALS ARMOR AND MAGIC RESIST PER CENT, so it makes you more tanky stealing his stats.
DONT FORGET, IF YOU BUILD RANDUIN, TO USE IT IN THE MIDDLE OF THE TEAMFIGHT. if you build the Ghostblade use it to run down toward the enemy first damage dealer.

Play defensive, dont push, last hit minions, if you think you can kill the enemy easily try to spam some bite to him, but dont waste all the mana! It doesnt ends easily but be careful, you can also use bite to last hit but just sometimes. and you dont need to use all the spells to harass the enemy champion, unless he attacks you obviously.
If you're low health don't forget your op passive, it heals you also from afar! you can just wait, but you will need to recall sooner or later. dont forget the combo/ulti/ignite, he can also kill escaping enemies.

Ignite is very good and flash is the best summoner spell in the game to escape. but also Ghost, exhaust and teleport are very good options.

TRUNDLE IS GOOD AGAINST
almost every tank and AD bruiser, due to his ad/resistances steal.
A real problem for champions like:
YORICK, TRYNDAMERE, LEE SIN, HECARIM, FIORA, RAMMUS, UDYR, NOCTURNE....

TRUNDLE IS WEAK AGAINST
Ap champions and almost every champions with ranged harassment, like:
KENNEN, HEIMERDINGER, a good MAOKAI, MALPHITE, KAYLE, RUMBLE, SHEN, TEEMO....