Well then why don't you do what far too many other players who use the imp do? Hide somewhere until their mech is available. There is an easy way Popcap could end this cowardly practice - make the cooldown system for the mech dependent upon action (like the legendary effect meters, not merely the passing of time).
And I can assure you, having played many a game of gnome bomb against half a dozen mechs, no imp is underpowered.
None of them.
