Forget all that jazz, you wanna play pure fire? Don't worry about healing so much, healing is for people who can't let go of nature's hand holding play style.
This works great on RPVE, using the various other buildings like mortar tower, makeshift and pyromaniac will help you with a lot of other Campaign maps
T1 get Mine and suppression <=== stops willzappers or even spawners and other annoying stuff
T2 = Firedancer or Fire Stalker with either Skyfire Drake or Blaster Cannon for anti air, Rallying Banner and lava field and possibly Wildfire (optional disenchant G to deal with freeze and paralyze on batariel or juggernaut to charge into camps) If you need a little extra sustain you can add in Ravage
T3 = Juggernaut and both Unity spells, shrine of war, inferno, On some campaign maps sniping targets with spitfire works great as well
T4 = Batariel, Fire Dragon and your choice of land unit, moloch for extra toughness paired with unity, Fire Worm for CC with its ability knockback and speed, or thunder wagon or magma fiend etc for a bit more micromanaging but aoe clear, Take Fire Sphere, Cluster Explosion (Infused) Earthshaker and optional Comet Catcher if you have the time You can also add in bloodthirst for additional support on batariel as it really keeps him going but I don't use it
Charge a juggernaut in (or any 1 unit) and unleash all your spells to kill the spawn building and clear most of the harder camps and "soften" your enemies, for easier camps just rush in your units with unity and their abilities build rallying banners along the way as rally points to summon new units at when the old ones are killed and you can also stop briefly at the banner to let your units recover
Only summon 3 or 4 units at a time no more then 5 as thats Unity's max and also so you have the spare power to Nuke enemies with your spells and sharing life points between all your units with unity will usually let them live just long enough to annihilate everything clean up whats left with explosions of various kinds.
And people think pure fire is hard (ﾉ◕ヮ◕)ﾉ*:･ﾟ✧ Just play aggressive and go for the most DPS and kill the enemy units first