First of, make a spell which your unit will use primarily (Acid Bomb-based with no damage over time and no AoE, just slow to target unit), then make dummy units with different Acid Bombs with unlimited range and almost-instant missile speed (max them values out)... make trigger that creates all those dummies at whatever point you want and order all of them to use their Acid Bombs on unit being cast on whenever your main unit uses its primary Acid Bomb ability... the trick comes now, btw if you don't want to have this spell to be missile-based use Slow or whatever without auto-cast, whatever, the trick is to set all those dummy units' Acid Bomb to be a little bit delayed, for example 1st Acid Bomb of 1st dummy unit will have 5s cast time the other 10s, the other 15s and so on with shorter duration every other skill... aaand it's done just make sure you kill or delete those created dummy units properly...
- this will cause the unit to be slowed by 10% for 10 seconds for example or what number suits you better, then the unit is hit after 5 seconds by another higher slow with also 10 seconds duration and again after 5 seconds, bam!
- the thing is that the unit's debuffs do not stack so you don't have to worry about having 1st Acid Bomb slowing effect + the other one but I'm not quite sure how it would work if two allies or non-allies cast their Acid Bombs on one mutual target, my guess is that those slow effects will overlap somehow but it won't matter much as the trigger will run smoothly at all times
- to protect this spell even better (from spell-shield or debuff stealing which is impossible because it's not really a stealable debuff) you can put created units that cast those Acid Bombs under condition "if unit being cast on has buff = Acid Bomb" or whatever...
This is tricky to make all those dummy units and their abilities but it's effective, If there's nobody with better solution feel free to use this or ask...