Alternatively, you could just swipe the 4 & 5 and you would still get a 6 left in place. But those 1 - 3 tiles would still be there.
The game ends when the board if filled with tiles.
Strategy: sometimes you want to swipe as many tiles as you can. Other times, you don't. For example, let's say you have 10, 11, and 12. At that point, I would swipe just the 11 & 12 because that will save me the time to build up another 10 all over again.
According to the game, you can get a 16 tile. The highest I've gotten is 13. This game reminds me of 12 - the Hardest Puzzle. However, Get Line plays much faster. If you ever get a 16 tile, please let me know!