You probably want to focus all potions on a single tower, and using a second one as support (Mr. Iron is the best choice).
Since the armor is different you have now to use two "main" usually I focus on one and once I get the pair of rings (yin/yan) on both of them.
Rings work after 20 seconds you equipped them. 1 tower has a ring and another one the second ring. Basically it means two towers can get benefit from potions instead of one.
Mr iron is used mainly as a support tower since he can store infinite items such as cauldrons and barrels which are useful.
By support tower, Manu means the support tower doesn't deal damage himself, simply amplifies the damage of the towers around it. For example, Herb Witch has low damage, but amplifies the attack speed (thus damage) of the towers around her. Similarly, Irish Pub has low relative damage, but amplifies the attack damage of the towers around it, and with the active, all the towers on the map.
Regarding your current set up, I'm not sure which tower is your "main" tower, the one that has been drinking all the potions and has the best items. In MazebertTD it's really important that only a few towers get the kills and level up. For instance, leveling up the Knux for more armour penetration is more important than leveling up the Herb Witch.
From your screen shot I can see the Helm on a Herb Witch. Right away I can tell you that's a useless use of the Helm. Like I said above, Herb Witch is a support tower, buffing the damage of nearby towers, not a carry tower.
Hope this helps. I think I'll write up a generic post about tower types ^^
- jhoi
@jhoijhoi your article tower typage was really beneficial and gave me new tricks and techniques at least I can make it to 200 and beyond in survival thanxx a lot. Did you get any strategies for shattered Plains??? I'm currently on it.
@istmander see first decide which will be your main and which will be support. Main towers do a bulk of damage while support looks after secondary actions like slowing down or aura effect etc.
Put Mr Iron in one of middle slots and give him loads of attack speed upgrades worth keys. Once your construct time is below 20 you can start giving him items like fist, barrel and couldruns. Another good thing which worked for me was to marry Ms Jilly to iron man and then stack up attack speed potions. Both got benefit and equipped ms jilly with luck items and she instant killed many creeps. Try improvising and read jhoijhoi's article. You'll get necessary information
Another interesting thing. Try putting bear hunter on middle squares where you've placed witch on corner square