First of all, I'll assume you are making the deck for Level 3.
I'd try to refine the deck a bit before going for upgrades. It seems very unfocused right now - it looks like as if you wanted to put in everything. The problem is, it doesn't work too well. That plague, for example, looks very random. You also have only 7 creatures, and even though there is the potential for more via mitosis, relying on drawing a single card is never good. 2 Empathic Bonds for only 7 creatures (potential 14 healing a turn, ignoring all the creature control that you might get in your face), in my opinion, does not worth it.
There are many ways to go with the deck. If you want to keep the Life/Death duo, I'd change the mark to Death, and switch out the Plague for another Poison, while adding a few more Emerald Pillars. This will make your draws more consistent - you will always have the quanta to play those poisons.
One way is to focus on the poison aspect. If you want this, use more scorpions, and either Mitosis or Adrenalines with them. Or if you don't like those, you can go for pure damage and drop in a few dragons. If you have an Arsenic or two, I'd also add them in in place of something. This will effectively turn your semi-stall deck into a rush deck. Your main weakness would be shields and creature control, since Scorpions are very fragile - a reason not to use Mitosis or Adrenalines with them. You will win and lose faster. (Note: if you want to try and build the decks yourself, don't look at the spoilers.)
52o 52o 52o 52o 52o 52o 5bs 5bs 5bs 5bs 5bs 5bs 5bs 5bs 5bs 5bs 5bs 5bs 5bt 5bt 5bt 5bt 5c2 5c2 5c8 5c8 5c8 5c8 5c8 5c8Heals allow you to get Elemental Mastery occasionally.
Another way is to focus more on the healing side, while waiting for the Poison to chip away at your opponent's life. You will still need other sources of damage, since 6 Poisons won't be fast enough. In this case, you need more healing, but I don't think Empathic Bonds are the way to go for this. If you have more Shards of Gratitude - great, but I doubt it. They provide cheap healing for every deck, so if you have them, you can use them here.
Hover over cards for details, click for permalink
52o 52o 52o 52o 52o 52o 5bs 5bs 5bs 5bs 5bs 5bs 5bs 5bs 5bs 5bs 5bs 5bt 5bt 5bt 5bt 5c2 5c2 5c2 5c2 5c3 5c3 5c5 5c5 6rn 8pkHeals, Druidic Staff and that SoG keeps you alive while the Poison wears down both your opponent and his or her creatures. Dragons help with the damage.
If you really like Bonds and Mitosis, you could try to build a deck around them too, but Life's creatures - except for Dragons - are not sturdy enough to survive all the creature control you will be facing.
About the upgrades.. Hodari is right. You can rarely, if ever, can go wrong with Pillars, since chances are you will need them again later for another deck. They hasten the deck considerably. If you don't like the idea of upgrading Pillars, I'd recommend you to look at cost-effect ratio. There are cards that gain a lot from upgrading, while there are ones that gain very little. Poisons deal 1 more damage when upgraded, which is very good, but they also cost 1
quanta more, which will slow you down a lot if you power them from your mark alone. I'd rather look at Scorpions or Dragons. Scorpions gain 1 more damage when upgraded, which means you can penetrate a lot more shields with them, while Dragons gain +2 | +3, for a 2
cost increase. Spine Carapace is good too, but there are times when you wish it wasn't upgraded: there are creatures with 1 attack (like Arctic Squid) whose main use lie in their abilities - unupgraded Carapace kills them, while the upgraded does not.
Okay, I'm off to sleep.
I hope I haven't missed anything and that this wall of text will help you a bit.
