You can make their health higher by having it a low percentage at the start and when certain conditions are met they heal more (gaining health).
In outpost rpg they do it like this
Conditions: (Heroes)
Current Player brings at least 1 "HeroA" to location "Heal"
Current Player's Death Count for Vespene Tank Type 1 is exactly 0
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Actions:
Modify Health Points for current player, set to '15'%
Preserve Trigger
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That makes starting health.
To make a unit get stronger (more health), you would do it like this
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Conditions: (Heroes)
Current Player Kills Score is at least 1000
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Actions:
Set Kills Score for Vespene Tank Type 1 to '1'
Subtract Kills Score '1000'
Preserve Trigger
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(this resets the experience of the unit'
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Conditions: (Heroes)
Current Player brings at least 1 "HeroA" to location "Heal"
Current Player's Death Count for Vespene Tank Type 1 is exactly 1
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Actions:
Modify Health Points for current player, set to '30'%
Preserve Trigger
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This increases the health. Of course you would need SCXE or Starforge to modify death counts for a vespene tank. (its a special unit)
Just keep setting the vespene tank kills higher until the hero unit is a full strength (Maximum Health Level)
For this to work, you would need to make a maximum health (around 500 hp) and an experience system. (Experience For Kills)
I've studied Outpost Rpg extensively, so i could provide further assistance.