Just a quick review of synergies, to explain which ones you should build towards and which ones you should avoid.
Death rounds: (+10 base damage, -1 max Hp) Death rounds is a fine synergy, adding base bullet damage makes it perfect for low base damage guns that you want to build into gun damage. Avoid it on the grenade launcher as your damage is already extremely high and the health reduction is a risk.
Frost fire:(Causes frozen enemies to burn) This synergy is completely useless as the Pyro mage upgrade which is required to unlock it gives your bullets a 50% chance to burn and the Frost mage only gives a 35% chance to freeze. This means that 99% of the time frozen enemies will already be burning. (and of course as i have said many times, damage over time is pretty bad overall)
Gun mastery:(15% bullet damage, fire rate and reload rate. +1 max ammo, +1 piercing) This Synergy is fine but definitely not worth working towards. Any bullet damage build will end up unlocking it at some point or another and it should probably be picked then as it is better than most upgrades.
Mini clip:(100% reload rate, 50% bullet damage. Max ammo of 1) The best Synergy in the game. This is simply due to its interaction with things like Fan fire, Ice shard and Aftershock which trigger upon reloading. If you want to be untouchable and a screen full of bullets this is the way to go. Almost always worth rushing to.
Overload:(Lightning that strike a burning enemy explodes for 25 bonus damage) For most of the game the electro mage and Frost mage trees should give you plenty of damage so i would’t recommend rushing to get this one. But one your build is set up dipping into the Pyro mage tree to unlock this synergy is always worth it.
Stand your ground:(+2 projectiles, +25% fire rate, -25% movement speed) This is an excellent Synergy. The problem is that it requires you to take siege which is completely useless. This means that Stand your ground basically costs you 2 levels. It is however extremely strong, so at some point if siege is among other mediocre/terrible options when you level up it is still worth picking up for the chance of getting this synergy. (The movement speed reduction is slightly annoying, but when you get this synergy you should be at a point where it is no longer a problem)
Summon Mastery:(35% summon damage, 35% summon attack speed, -35% Bullet damage) If the name wasn’t a dead giveaway, this synergy will (or at least should) always be available to any summoner build. The 35% bullet damage reduction is irrelevant ant the 35% attack speed for your summons is absolutely terrific.