Frontliners are built on distraction and defence.
Your priority as a Frontliner should be to sit on the point.
The sheild that all Frontliners come equiped with should be used as much as possible to protect you and your team.
Frontliners can soak up the most damage in the game. (Except for Barik)
Frontliners are the most important champion in a team. If there isn't a Frontliner in your team, then it might just be your responsibility to help the team through.
All you need to do as a Frontliner is sit on the point and become the enemies vital target.
If this is achived then your team's Damage Dealers can take down anyone who poses a threat to you.

Damage Dealers are made to stick behind or near the team's Frontliner.
Having more than one Damage Dealer is never a bad thing.
Avoid drawing too much attention to yourself from the enemy team.
Keeping the Frontliner alive should be one of your priorities. (So that the enemy team targets them and not you.)
Damage output (Really? I never would have guessed that.) should be your main focus, with the enemy team locked-on to your Frontliner, you can get some serious damage out there without needing to fret too much about becoming a target for the enemy team.

Stay close to your team's Frontliner, you scratch their back, they'll scratch yours.
When healing teammates, the Frontliner should be the priority, if he's fine, then aim for a Damage Dealer, or someone else who needs it.
Damage should be dealt against threats that could endanger the Frontliner. I'd reccomend keeping a good look out for Flankers, the Frontliner needs to protect themself from the Damage Dealers.
Stick with the Frontliner. Heal the Frontliner. Protect the Frontliner... And he'll protect you and the team in return.

Flankers can deal ridiculous amounts of damage in short periods of time.
Flankers are also super fleshy.
As a Flanker you should do exactly as the name suggests, Flank the enemy.
Find a path around all the action and then ambush an unsuspecting target.
Putting pressure on the Damage Dealers should be your goal, if you're close to death, RUN. Come back again not long later once your health has regenerated.
Hunt down the enemy team's Damage Dealers and distract them from the Frontliner. By killing them, or putting serious pressure on them instead. When you think about it, you're the Distraction's Distraction.

Let's pretend both teams are using the usual set of Champions: 1 Frontliner, 2 Damage Dealer, 1 Support and 1 Flanker.

The Frontliner needs to distract the enemy from your Damage Dealers so that they can kill as many of the enemies as possible whilst the Support Champion heals the Frontliner so that he stays alive longer, thus allowing the Damage Dealers to stay (Mostly) safe for longer. And behind enemy lines your Flank Champion can mow down enemy Damage Dealers so that the Frontliner stays alive even longer and continues to play as a distraction so that then enemy team doesn't attack your team's Damage Dealers. Make sense? I hope so.

If you have any ideas/constructive critisism then feel free to comment them below.