Well... This is right up my ally!

Ok:
Let's say you have 5 levels
Conditions:
Always
Actions:
Set Unit HP to "X"%
Set Deaths to 1 for unused unit for current player
Conditions:
Deaths = 1
Player Brings "unit" to "Y" location
Actions:
Set Unit HP to "X"%
Preserve Trigger
Now, you'll do your leveling up system. However you do this is up to you.
Level 2
Conditions:
Your Conditions to allow the next level too be reached.
Deaths = 2
Player Brings "unit" to "Y" location
Actions:
Set Unit HP to "X"%
Set Deats to 2 for player
Preserve Trigger
Level 3
Conditions:
Your Conditions to allow the next level too be reached.
Deaths = 3
Player Brings "unit" to "Y" location
Actions:
Set Unit HP to "X"%
Set Deats to 3 for player
Preserve Trigger
Level 4
Conditions:
Your Conditions to allow the next level too be reached.
Deaths = 4
Player Brings "unit" to "Y" location
Actions:
Set Unit HP to "X"%
Set Deats to 4 for player
Preserve Trigger
Level 5
Conditions:
Your Conditions to allow the next level too be reached.
Deaths = 5
Player Brings "unit" to "Y" location
Actions:
Set Unit HP to "X"%
Set Deats to 5 for player
Preserve Trigger
If you can see. Each level, the death counter is set to one level higher. I.E. Level 1 = 1 death, level 2 = 2 deaths etc....
So, what you'll want to do is, every level add 1 death. That way, you save switches. Also, how you get to level up is up to you. I am just showing you what happens after you have leveled up.
If you don't understand, let me know. I'll try and elloborate more.
ADDITION:
I was posting while UED77 owned the topic. So, you'll probably have the concept down after reading both of the replies.