http://www.staredit.net/index.php?tutorial=124 - Tutorial on Death Counters.
If no other trigger uses wait actions, try this:
Conditions:
-- player wants to use grenade
Actions:
-- remove a grenade
-- Wait for 5000 milliseconds
-- Kaboom!
-- Preserve trigger
If you really wanted to be economical with variables (i.e. to use as few death counters and switches as possible), and you have wait actions for players already, here's a modified version of Ultimo's system:
Conditions:-- player wants to use grenade
-- Current Player has suffered at most 0 deaths of unused unit
Actions:-- remove a grenade
-- Set deaths of unused unit to 1 for current player
-- Preserve
Conditions:-- Current Player has suffered at least 1 deaths of unused unit
-- Current Player has suffered at most 59 deaths of unused unit
Actions:-- Add 1 deaths of unused unit for current player
-- Preserve
Conditions:-- Current Player has suffered at least 60 deaths of unused unit
Actions:-- Set deaths of unused unit to 0 for current player
-- Kaboom!
-- Preserve