This build is designed to shorten the gap between you and everyone else, whilst applying utility to your abilities to allow you to hit that ever so damaging ult, all whilst keeping the tanky attributes often given to ymir. I warn you this is my first guide so it will most likely be bad, but please.. make of it what you will, and I hope it helps in some way.
This build is especially good when you begin to take advantage of ymir's passive early/mid game, using the slow from all abilities and the gem of isolation you can start clubbing people like there's no tomorrow and deal some decent damage.
Use Ymir's Ice wall to aid yourself and your team, you can also use the wall to escape or shut off members of the enemy team. The wall is best used when 'funneling' opponents into ideal positions for your team and to cut them off from any means of help/escape.
Ymir's glacial strike is very powerful, especially at the lower levels, this skill should be used to deal decent damage to the enemy team's squishier gods, it's found effective when combined with the ice breath to guarantee a successful hit on the opponent.
Another one of Ymir's utility abilities, this skill can be very useful to shut members of the enemy team off and leave them as an easy target for your team to pick up the kill on. Best used to deal with the damage dealers on the enemy team, although can also be used as a form of escape or to save a team mate.
Shard of ice:
Ymir's ultimate is arguably to hardest of ultimate's to actually get decent hits with, the timing of the ult is key, and takes some getting used to, however if used in combination with the likes of Cupid/Hades/Odin/Artemis it can be very effective, it can also be effective mid fight when you know that the enemy team's escapes have been used to initiate the fight. When all else fails Ymir's ult can be used as a last ditched attempted to dish out some damage before you go down, as long as you give yourself enough time to cast it that is.